Audio Research VS110, VS115?

Adrian, I currently own a D70 Mk2 and your memory of your old amp is accurate.
In fact, with current gear this amp is IMHO still very close to SOTA. I have actually AB'ed the D70 ( Ok I did roll in some NOS tubes) against a VT 100Mk3 and our group all preferred the D70 that night! The VT has more power and can drive the speakers a little easier in the bottom end, BUT that is were the superiority ends... The D70 Mk2 has one of the best midrange reproduction and ability to portray air and space of any amp that we have had in my system, easily outpacing the VT 100Mk3 that night!!
In fact one of my 'phile friends is selling his VT 100Mk2 to replace it with a D70 Mk2.
Amazingly, another 'phile friend who owns the Ref110,and who raves about that amp, likes the D70 Mk2 enough that he is planning on using one as a swap out with his Ref 110 on certain music!
So, since you can acquire a D70Mk2 at a give away price right now, you might want to re-acquaint yourself with an old friend.

Adrian, That's a very good question about the biasing of the D70..
Frankly, this is perhaps the only real drawback to the amp. The procedure is NOT for the faint of heart..:0(
I just re-tubed my D70 with rolled in tubes and there was NO way I was going to do the biasing myself. According to the ARC manual it is a procedure that any 'phile can do....BS!! Who ever wrote and thought that at ARC back then,must have been smoking something!! Nonetheless, once my tech had the amp biased, it hasn't wavered since and the sound is to die for. Luckily, i have a couple of very good tech's near me; if that isn't the case for you, then perhaps an auto-biasing ARC amp would be more appropriate. (Although, I think that the sound of these is inferior)

Adrian, That is a very difficult question to answer... it probably depends to some extent on the quality of your tubes and the amount of turn on/turn off's that the tube cycles. Also, possibly there is a factor of how much ventilation etc. that the tube sees. In my case, I have had no drift of the bias setting after a couple of hundred hours, but i couldn't tell you how long that will last for.
I think that a more appropriate question would be how much is the typical re-biasing cost for a tech to do the work once the bias has floated or new tubes need to be inserted. That answer probably depends greatly on who is doing the work and again I would suggest asking questions of a nearby tech if you have access to one.