Autoformer versus Transformer Passives?

Any practical and/or theoretical advantages to one apporach for passive preamps? I just got a BENT TAP-X using autoformers and I think it is quite wonderful. Any passive users out there with experience with various apporached to passive preamplification?
Well, technically (I think) the only advantage supplied by transformers is galvanic isolation from sources passing DC to the amplifiers. But, if DC is not an issue, then the single winding of the autoformer is simpler, more direct, and ultimately "purer" than the two windings of a transformer.

In theory...
I think the isolation is the one theoretical advantage. There does seem to be less "stuff" in the autoformer which would seem to be a plus. I wonder if this bares out in listening impressions. All I know is the TAP-X sounds great, but I never was able to compare it head to head with a TVC. When I bought the TAP-X I was given the option to get it with the S&B tranformer or the Slagle autoformer and chose the latter simply on the grounds that in the passive world less should be better as long as you are addressing impedance matching effectively.
I have an S&B TX-102 TVC and it is great when in a properly matched system. Worked beautifully between my upgraded Museatex Bitstream dac(4V output) and former Art Audio PX-25 & Welborne DRD 300B SET amps. But, couldn't use it between my phono preamp and amps.

I remember when Dave Slagle first started winding(or posting about winding) his own transformers for his DIY SET amps. He and Mickey from Magnequest transformers would have long, really interesting discussions about all the factors that go into transfos on Audio Asylum. Then, he started making autoformers and I have been really intrigued ever since.
I had an S&B MKIII based passive made by K&K, but it was too long agon and the system context was different. I got the Bent based on user comments here and Teajay's review of the TAP. There seems to be the view that the Slagle autoformer sounds better the S&B transformer, but I've not seen much comment on Audiogon. If a preamp is suppose to be neutral and simply let the source to the amp with attenuation, it does seem passive setups with sufficient gain to drive the amp would function that way. I would like to see a shootout with something like the BENT-TAP-x versus some of the 10K and up active tube preamps. It does seem that a passive that can control the interconnect capacitance should be the ideal, with the amps adding whatever color or flavor the listener prefers.
Your (previously my) K&K used S&B MkI transformers, not MkIII. The difference was the MkI version tended to sound a little warmer.
Hi Anthony. I stand corrected. Have you heard any of the Slagle transformers stuff?
Working on getting the new Slagleman's. These are sold by Intact Audio. Sort of a "nude" version of a non-remote TAP-X. I'd like to compare them to my Lightspeed and hopefully can remember enough about the S&B MkI's to make a reasonable comparison. I'll let you know the results when I get them and have had enough time to listen.
I have spoken with Dave several times and made a few autoformer preamps for friends, using his and Pieter's autoformers.
Transformer passives have more bass distortion as compared to autoformers. This is easy to confirm: look up the low frequency response graphs at S&B and Sowther sites, also, Jensen line input transformer data sheets, including % of harmonic distortion at low frequencies. A magnetic core will saturate and distort with a combination of very low frequencies AND high input signal levels.
OTOH, a transformer's galvanic isolation is great to get rid of ground hum issues, it also attenuates very high-frequency digital noise such as the normally unfiltered SACD outputs or out-of-band DAC artifacts.
IME, autoformers have great bass, outstanding headroom and can be made to roll-off smoothly at ultrasonic frequencies.
In summary, IMO, if your system has ground-induced hum, or an SACD player or your CD player outputs digital HF nasties, use a transformer volume control.
IMHO, if your sources are clean of HF noise and there is no ground-induced hum, an autoformer is a superior solution across the board: bass, dynamics, low level detail, frequency response, distortion AND musicality.
My 1c worth (adjusted for recession)
I asked John Chapman (BENT) about the ground-induced hum before I chose the autoformer over the transformer (he had a few S&B still available)and he said that in most cases that should not be a problem - I hear no hum so I imagine I'm safe on that ground. I'm not hearing any obvious HF nasties from my Accustic Arts DAC so hopefully this is a pretty good application for me. I would love to have A/Bd them but that was not possible.
I think I've mentioned this before, but I built my own linestage using S&B mk3 transformers from John. I tried wiring them up as autoformers, as mentioned on some forum. There was a very slight difference in sound but, in the end, it was more important for me to use balanced connections with higher gain. It would take 4 autoformers for balanced I/O.

The autoformer wiring did seem to have a bit more deep bass, all other being equal. I also have the ability to turn up the gain on my bass amp to account for that. My Genesis 350's are not lacking for dynamics and bass but would certainly reveal if there was an issue in the chain. Easily (and queezily) capable 0f 20Hz test tones.

Comparing the Slagle autoformer to the S&B would be a different story.