Audio Research V35

 The difference is negligible. Keep in mind that the V35 came with KT90 tubes. So if you use 6550 tubes the output will be slightly less. Also the bias pot range is tight on both the Classic and V series. You might have to change a couple of resistors if you want to run 6550 tubes in a V series amp.

The V series is basally a balanced version of the Classic amps. Some of the very last Classic amps were also balanced. I have owned both the Classic 60 and V70. These are two of my very favorite ARC amps. I wish I had never sold my V70. Just wonderful sounding amps.

The tubes are KT 90’s. Not sure what the sonic advantages and disadvantages are between these and 6550’s are. 

I bias my Classic 60, a pain, but not terribly difficult with the right probes a patience. 
I have a BAT VK-3ix balanced preamp, so it should be a good pairing. My Classic 60 sounds good with it, but no output impedance is indicated on the spec sheet of the V35, but I’m sure it will be fine. 

The bias points on my V70 were a little different than the Classic 60. They were in front of the power tubes near the transformers. I had a couple pairs of probe extenders that I clipped onto the bias points before I turned the amp on. Otherwise you have to reach in between the power tubes and transformers. I would imagine the V35 is the same way.

 KT90 tubes were very hard to come by when I owned my V70. I always ran 6550 tubes in it. New manufacture power tubes of all types are a lot easier to find today.