Audio Research REF 75 vs VAC Phi 200

Vac is a much better amp!!

I have no view as to which amp is better. I would caution you though that the amps may have different electrical attributes that may affect speaker compatibility. Specifically, output impedance, damping factor, and power supply size. If possible, try to audition the amps with your speakers.

The AR has very few rolling opportunities with the KT120s and 6h30s, pretty much stuck with very few choices unlike the VAC.
One positive, I could drive the AR to manufacturer for service.

Both are excellent amplifiers, you really can't go wrong with a choice. I prefer the VAC to ARC, but it's really personal preference in the sound I like (I've always felt ARC was more tilted to the highs, even their newer models, and I like a full midrange, which I definitely get from the VAC). As far as tube-rolling, while you certainly can do it with the VAC, I haven't had the urge to do so, which is unusual for me.

One thing that Bifwynne points out is a definite consideration with the VAC, as it is very sensitive. I have a Shindo preamp with a lot of gain, and I could barely get the volume control to 7 o'clock before it was at a comfortable volume. I actually have put in attenuators at the amp (something that is built in to VAC's Phi 300.1) to get a little more play in the Shindo volume control. On the other hand, using a TVC passive (Promethius) as my preamp has resulted in no such problems, so you may need to consider the output from your preamp in making your choice.

Actually, based on correspondence I have had with another member about his VAC tube amp, I understand that his amp has a variable negative feedback (NF) control. As is usually the case, NF often times reduces an amp's output impedance and correlatively increases the amp's damping factor (DF).

Ok ... where I am going with this point is amp/speaker compatibility. As I wrote in my post above, I am not familiar with VAC amps. However, if they use little or zero NF, then an inference is permitted that they also have a "high'ish" output impedance and a "low'ish" DF. These attributes could result in acoustic colorations if the speaker's impedance characteristics vary considerably as a function of frequency.

In the case of the member mentioned in the first paragraph, his speaker choice was quite deliberate. Namely, one having "gentle" impedance characteristics that make for an easy to drive load with a minimum of acoustic coloration.

By contrast, most ARC amps use more NF and consequently have "low'ish" output impedance and "high'ish" DF for tube amps. As a result, this class of tube amps performs somewhat "SS-like" and can couple with a broader range of speakers.

That is the longer version of what I had in mind. I try not to be a name dropper. I'm sure the member I referred to above knows who I had in mind. If he wants to chime in, he should of course feel free.