Audio Research REF 75 vs VAC Phi 200

I don't think you can go wrong with either of these amps. I personally like Vac equipment a lot and have found that Kevin Hayes of Vac is a really great guy to deal with.
Alan

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.