The VT100 mk3 is not a bad amp. I owned a mk3 for a few years. I never had any problems with it other then a tube blew a resistor once, not a big deal. I bought it used from my ARC dealer because I needed to send my V70 back to ARC. My plan was to probably keep the VT100 and sell the V70. I never could and I ended up keeping both amps. The VT100 was more detailed but sounded a bit thin next to the V70. It also lacked much of the midrange magic of the V70.
I now own a Ref 110 which is much better than either the V70 or VT100. The Ref 110 is far more detailed then the VT100 with the midrange magic the V70 had. And KT120 tubes make this amp even better.
If I were looking for an ARC amp right now I would strongly consider the Ref 110. The price of this amp has really dropped, especially right now with the introduction of the SE amps. KT120 tubes are a drop in replacement and really adds to the sound. ARC advises against using the KT120 tube in the VT100.