Anyone had the equipment customized ?

To improve sound and readability, of course. Besides vintage turntables - this is widely done.
Yes; kinda, maybe, sort of.   Being a electrical design engineer, I design a log of amplifiers and analog control electronics at work.  So naturally, I do this at home with some of my equipment.  For a while, I was doing custom electronic design for folks who had high end stereos. 

The key is finding where the weak spots are in the equipment, then making those spots more robust.   By weak spots, I mean where the designer left performance on the table and didn't pack it into the product.   Anyone can tweak stuff, which may or may not sound better, and may not give you much bang for your buck.   And, trust me, hiring someone to do custom engineering on your equipment is expensive. 

If you go this route, pick someone who routinely modifies your brand and model often.   That is, they have a long history of fixing that particular model.   Ideally, they should be someone who actually worked at the factory or someone who worked in their service dept.  If they don't meet this criteria, don't send them your equipment or money!

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I have had Steve McCormack mod and upgrade his equipment, which I would consider 'customising', as I can pick and choose what I want done.

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My tube amp and linestage were from a builder that only does built-to-order units.  For the linestage, I could specify whether it would actually include a phono-stage, whether the output would be capacitor coupled or transformer coupled (or both options available), the type/brand of input and output transformers, etc.  I chose a preamp (includes phono-stage), transformer inputs and outputs, I asked for remote control of volume (something the builder had not offered before), and I requested a balance control (implemented by utilizing two big Davin controls).  The custom amplifier has matching/complementary transformer inputs.  I initially got the amp with Langevin input transformers, but later, I went with Western Electric input transformers (the amp is essentially a rebuilt Western Electric 133a amplifier).

My speaker has been modified by me.  The midrange horn and compression driver have been replaced by a vintage Western Electric horn and compression driver.  This was easy to do because the horn in my system sits on top of the woofer cabinet on a totally open platform  The replacement horn actually looks better than the original because its 24" width exactly matches the width of the woofer cabinet (the original horn is smaller and weird looking).  I added an L-pad attenuator to control the output of the horn driver.  I will be doing additional modifications, including a totally new (but made out of vintage parts) crossover and top notch wiring for the speaker (probably Audio Note Sogon wire).
I suspect there’s a lot of overlap among customizing, modification, upgrading components (like capacitors and fuses) and tweaks. I once owned the World’s most extensively Modded Oppo 103, trust me, you wouldn’t have recognized it. After Ric at EVS modded it I really went to town on it. You can probably guess some of things I did, beyond what Ric did, maybe not. Some things I did, however, were definitely beyond the scope of this discussion. Perhaps, beyond the pale. The deeper you go the higher you fly.
" the deeper you go the higher you fly ". Right. But you also got to land safely.
Larry, that's extensive modifications/customizing. Impressive.
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Back when I was much younger and was just starting to build a system with separate components with my first pair a Magnepans, I couldn’t keep my hand off the soldering iron. I would mod everything and with the modest equipment I had in those days it improved what I had. Did several mods for friends that actually had much better gear than me at the time.

With what I own today I would never dream of modding my equipment now, and honestly it sounds so good I’d be afraid I’d mess up the sound. Modding today for me is cables, isolation devices, room treatment, power receptacles and etc. Used to build my own cables when I could build better than I could afford, but again happy with what I have. One thing I have done inside my gear is change out the fuses but that’s as far as I’ll go.

Oh, if I picked up an old Adcom amp, it’s game on.
I had Noble Electronics modify a DAC. It came back a different unit, sound much improved. However, the money paid for these services is almost never recaptured when you sell the unit. In my case, about $1000.
When I was modifying equipment and making money at it, I found some horrible stuff that came in from folks who someone modify it but didn't know what they were doing.   One crossover came in the shop and the guy used acid core solder on all the connections.   Another had such bad workmanship, I had to rebuild the entire thing and fix it before I could modify it.  

If you go this route, make sure you go with someone who knows their way around an electronics lab and has impeccable workmanship.   Take a gander at some of the work they have done in the past and get recommendations.
The problem with some "modifiers" is that they simply "upgrade" parts by substituting more expensive and currently trendy parts without considering the impact on sound.  Some parts choices, particularly with higher end gear, is determined by voicing the component, not just by picking the cheapest thing that will work.  It is not automatically the case that a more expensive part will sound better.  For example, I spoke with a builder who said that in his designs, the expensive, super tight tolerance Vishay resistors sound terrible, and I know a couple of builders who also prefer "cheap" carbon composition resistors.  Same goes with caps.

A local dealer who employed a builder to make his own house brand of tube electronics once asked me to listen to one of his amps.  To me, it sounded bad compared to other amps he had built for him and I got up the courage to tell him that his amp sucked.  He was actually happy because what he did not tell me ahead of time is that this amp had been sent to a modifier by a customer who did not like the result and wanted it fixed.  The modification involved Blackgate capacitors and teflon caps, etc.--all the latest rage.  

That does make a lot of sense.  Rail to ground decoupling caps work much better at killing noise when they have a moderate amount of ESR in them while DC Blocking caps passing a signal work best when ESR is held to a minimum.  
I’ve upgraded my phono stage with a Burson SV6 Classic and regulated power supply. Soon to modify my integrated amps to bypass the integrated phono stages. 
@spatialking said:

Rail to ground decoupling caps work much better at killing noise when they have a moderate amount of ESR in them while DC Blocking caps passing a signal work best when ESR is held to a minimum.  

That is incorrect. ESR in bypass capacitors reduces their effectiveness. The goal of bypass capacitors is to stabilize Vcc and displace noise to a part of the power bus that has higher impedance than at the point of consumption. It is  the lower impedance in a voltage divider. The lower that impedance is with respect to the upstream part of the supply rail, the more noise is displaced there. 

That is the often thought philosophy.   It is a small amount of ESR, which dissipates the noise energy as heat, rather than dumping onto the ground plane and causing ground bounce.   Sure, a cap with large ESR can cause a spike on the chips power pins, what is needed is a small amount of ESR in a bypass cap to dissipate that spike as heat.     

Some large computer back planes have shunt VHF caps in series with a 1 to 10 Ohm resistor to ground just to keep the power plane resistive and lossy at higher the frequencies where typical bypass caps have gone through their internal self resonance.  

For series DC blocking caps, then, yes I totally agree, the lowest ESR cap is best.   
High ESR means that the terminals of the cap will not present a low impedance to high frequencies. Since bypass caps are in parallel with the Vcc pin on ICs, that means the noise is still present. The only way to displace the noise is to present a much lower impedance than the power rail etc, that is upstream from the point of consumption.