Using step up / down transformers?

Is there any sonic or functional detriment to using a step up or step down transformer to run a Japanese or European amplifier at North American voltages? If not, doing this would open up a lot of opportunities...
No problems! Get one with enough current/amps rating for the device! EBay has plenty available! Along with foreign AC plug adaptors.
I'm using a 120/100v transformer for my JVC TT81.

I got a decent quality, not too expensive, small transformer, to make it easier to locate it behind the TT away from other devices.
There's a reason transformers are the heart of many power conditioners: they just naturally filter RFI. So not only is there no detriment, there is a benefit.
Nope.  As others have written though, make sure to size the transformer appropriately or oversized. :)

Not too much oversized though.  30% more VA than you need is a good amount.
Depends on how resolving your system is. I built a custom bucking transformer some time ago to reduce over voltages coming into my home. Unit was built to audiophile standards, with high quality transformer,matting to minimize vibration, emi/rfi shielding, quality internal wiring, oyaide R1 ac outlet. Only my Art Audio Carissa Signature 845 SET amp was plugged into this.
Eventually, over voltage coming into house was eliminated, bucking transformer eliminated. Immediately, I heard superior transient and micro dynamic presentation. It may seem counterintuitive to think a transformer would decrease sound quality, after all, they're used in much of our audio equipment, still, I hear this degradation. In my experience, transformers on AC lines have degraded the performance of all my tube amps, push pull or SET, high powered or low powered. Same degradation of transients and micro dynamics on every amp. This also goes for all transformer based power conditioners.

I'm not alone in hearing degraded transient response and micro dynamic loss with power amps on transformer based power conditioners, some have issues with macro dynamics as well. Some may blame other components within transformer based power conditioners as the culprit, specifically capacitors. Well, I've upgraded caps within these pc's with the best highest voltage capacity film caps, still hear the degradation.

And then, most of these step up, step down transformers you see are not near audiophile quality. And so, while one may save much money buying other countries' domestic market products, a quality transformer will minimize or eliminate any cost savings, and could possibly increase expenditure.

I had thoughts of doing same thing you're contemplating some years ago, some pretty substantial savings to be had,  Line Magnetic amps and Technics SP line of turntables come immediately to mind. I wouldn't purchase any amp in this manner based on above argument. My issue with non-amp equipment is the mostly cheapo step up/step down transformers, and the warranty and service issues. If new product, good luck with that, used, I doubt any tech will work on it. I do see a single instance in which I would purchase in this manner. Lets say you have a certain magical piece of equipment not available in US, big buck purchase, you can find a US tech's assurance they'd work on it if any future problems (or assurance of country of origin tech), don't care about warranty if new. You have the ability to diy audiophile step up/down, or find a tech to build audiophile quality step up/down, or find one off shelf (good luck with that, I never found one when building my bucking trans).  I'd say go for it. For myself, I'd always wonder if I was losing some performance potential on any equipment other than amp, amp no way. Still, may be worthwhile if you find that particular magic item. None of this hassle worth it to me unless it is extremely costly and unique piece. We've got plenty of special equipment available here, why go this way, trying to save a buck is not a reason to do this.
I'm only giving this long winded answer as I've contemplated doing same as you. All the above, and more have dissuaded me from pursuing some pretty amazing deals. If you don't believe me, ask yourself, why aren't more doing this, I presume they've come to the same conclusion.
I want to add that one experience that I had using a step up transformer was that my line voltage is 120 volts or so, and then measured AFTER the transformer it was 240 volts. I suppose that makes good sense BUT, the Cayin CD22 CD player being used did not think so. It accepts 220 volts, and with less for a margin/deviation than what it was given. Bottom line, repair of the CD player was needed.
My fix was to either use variac in front of the step up transformer, or use a PS Audio power plant in front of the step up transformer. In both cases I would adjust the voltage feeding the step up transformer to about 110 volts. This of course resulted in the CD player getting 220 volts.
The good news is that the player sounds so great, all of this fuss is worth it. Maybe there is a step up transformer wound a bit differently on the secondary that would make this a non issue in the first place.
I agree any transformers on the main AC power lines feeding your audio equipment will degrade sound quality. That is the reason why I run all my audio gear on 240 volts. I have 4 dedicated audio circuits in my audio room. Two of those circuits are dual voltage plugs meaning the top of the plug is 120 volts and the bottom is 240 volts. The plug is a Leviton 5842-I. The other 2 circuits are a Leviton 5822-W plug. All 4 circuits are 20 amps. Removing the 240 volt step down transformer from the main AC power line and running everything on 240 volts direct was the biggest improvement in my audio system. Improvements heard in the system are the following; better low level detail, dynamics, transient response, lower noise floor, micro dynamics, and the hiss from the speakers was eradicated. Also, I plug all my audio equipment directly into the plug. Switching from 240 volts to 120 volts in my system is now as easy as changing a power cord. Every time I switch my gear from 120 volts to 240 volts the latter is best sound quality. And that is my story.

BTW, 240 volts, is more efficient then 120 volts, 240 volts, is half the amps as 120 volts, 240 volts is quasi balanced power.

Running my system on the 240 lines already in my house has been a recurring thought.
any transformers on the main AC power lines feeding your audio equipment will degrade sound quality. That is the reason why I run all my audio gear on 240 volts ...
Your power went through multiple transformers as part of the electric grid's distribution system before it reached the transformer in your neighborhood. There's no way to be rid of them. Your audio components also contain transformers.
BTW, 240 volts, is more efficient then 120 volts, 240 volts, is half the amps as 120 volts, 240 volts is quasi balanced power.
No, the two have the same efficiency - the current is the same at either voltage.
Not too much oversized though. 30% more VA than you need is a good amount.
This statement isn't quite correct. AC power transformers will generate distortion, particularly if loaded past about 50%. So to prevent the transformer from distorting the AC line waveform it really should not be pushed past 50% of its total rating.
GOOD transformers will not degrade sound. +1 millercarbon, there is a reason they are still used today in audio. I always double the power requirement specification of the equipment to get the power rating of the transformer.
Sorry I was not clear:
I know the power goes through multiple transformers as part of the electric grid's distribution system before it reaches the transformer in my neighborhood. I know my audio equipment has transformers. I meant by adding transformers to the main AC power lines/branch circuits, from the electrical panel in my house to the audio equipment in my room. would be sound degrading. i.e. step up / down transformers, bucking transformers, power conditioners, AC regenerators (none of the former are on my audio lines)
Yes, my audio equipment sounds better on 240 volts. I plug all my audio equipment directly into the plug. That has been my experience. Audio manufacturer, Emotiva thinks audio equipment sounds better on 240 volts. See the link below. Page 10 section 7 regarding IEC power cord.
BTW, 240 volts, double phase is more efficient then 120 volts, single phase. Maybe ask Jim jea48, about voltage and amperage. Maybe he can explain to you better then I or show me where I could be wrong. Mike

I am using ACUPWR step up transformers to run 230v subs /CD/DAC. Heavy duty build quality. Sounds identical to native 230v when the components were used in Europe.

... 240 volts, double phase is more efficient then 120 volts, single phase. Maybe ask Jim jea48, about voltage and amperage ...
No, 240VAC is not more "efficient." Your equipment will operate at the same efficiency at either 120 VAC or 240VAC - and it will consume the same amount of current either way. Feel free to measure it for yourself.
I'd like to be more precise in regard to transformers on ac lines. I stand by what I said about diminished performance with my diy bucking transformer. I did suffer transient blunting and micro and macro dynamics suffered with trans. I'd like to make clear this was with 845 SET amp, this amp generates around 1000 internal watts and requires much current. I use dedicated ac line with 10 gauge special romex to feed. I've also  found my 845 amps or any of the other many tube amps I own do suffer from these same liabilities using any power conditioner, regardless of design, and I've tried many over the years.

Now, I do use a transformer based power conditioner on the rest of my system, this 82lb behemoth diy modded with high voltage film caps and Oyaide R1 receptacles. This has proven to be best pc for all the rest of my equipment. The amps do suffer with this pc.
IMO, amps should be on their own dedicated 10 gauge ac line, nothing else on that line.
''IMO, amps should be on their own dedicated 10 gauge ac line, nothing else on that line.''

I agree 1000%. 
Let's say that a 120 volt line (load) is drawing 10 amps. Since it is a single ended design, the draw will be carried by the hot conductor. 
 By the same token, using the same example of a 10 amp draw, a 220 volt line will still draw (or I should say the load will draw) 10 amps total, BUT since it is a Balanced design, each conductor would be drawing 5 amps. The total is the same. The word efficiency, may be applied to the fact that a smaller gauge wire can then be used. 
 I have heard however that there is an increase in efficiency of electric motors because the power is coming at opposite phases (times).
Remember: Volts/amps.
Maybe this will help.

P = I x E
P = Watts, Volt-amps.
I = Current, Amps.
E = Voltage.

Will a 240V 1500 watt space heater connected to a 240V source produce more heat than a 120V 1500 watt space heater connected to a 120V source? No.....
Current, amps?
P / E = ?
1500W / 120V = 12.5 amps.
1500W / 240V = 6.25 amps.
Remember: Volts/amps.
Maybe this will help.
@ditusa you are confused. 240VAC is not more efficient than 120VAC. You apparently believe than amperage is a measure of power, which is mistaken.

Power is measured in watts. To calculate power, you multiply the amperage (which only measures the flow of current) by the voltage.

You’d probably benefit from reading a basic text in how electricity works.