I believe Shane Buettner sold his original VTL7.5 to buy a REF3 about 1.5 years ago. I chatted with him when he had his VTL up for sale on AG and I know he ended up with a REF3 shortly thereafter.
I bought a brand new VTL 7.5 and used it for about a year. During that time period, I borrowed a REF3 (the upgraded version) from a local shop and used it in my system for about a week. I was using a Reimyo CD Player & Vandersteen 5A Speakers. I felt that both preamps produced wonderful sound, but I preferred the VTL 7.5. In my system, the REF3 added noticeable bass boost and seemed to just slightly add exaggerated sparkle on the top end. I preferred the softer sound of the VTL that I would describe as more accurate and/or natural (assuming such a thing really exists outside of my head).
Additionally, I had spent SIGNIFICANT time tweaking/fighting the VTL and found running it as VTL recommends works best. Valhalla PC directly to the wall outlet and sitting on a good isolation rack. I used the VTL for several months running into a Nordost Thor and this proved to be a bad thing. I mentioned this to the Nordost guys once at a demo, that convo did not go to well
I found that running the VTL into the Thor (and 4 other power conditioners I own) produced a very noticeable cold, flat and pretty much lifeless sound. I have heard many guys here on AG describe the VTL exactly that way and I wonder if the units they were using were running into power conditioners.
With the REF3, I plugged it directly into a wall outlet, using a mid level AudioQuest PC. I did not play with it anymore then this and I would agree that it would be possible to make subtle changes to its sound via tweaks, so yes, your mileage will vary.
About 10 months ago, I bought a new CD Player that has a built in volume control, a highly modified Esoteric UX-1. After a very short time, it was obvious that this Esoteric player sounds best with no preamp and thus I was able to sell my VTL 7.5. I really liked the VTL 7.5 while it was in my system and it worked 100% flawlessly from day one, which I can not say about the majority of high end gear I have owned.