Audio Research VT100, MKii and MKiii

Anyone have experience with the different versions of the ARC VT-100? Upgrades worth the extra money? Thanks.
I have listened to both the mk 2 and mk3 versions and I have owned the mk3. Stay away from the mk1. The mk 2 and mk3 sound a little bit different. But I would not say there is a big difference. The mk3 is a little more detailed but maybe not quite as lush. The mk2 uses 6922 tubes while the mk3 uses 6h30 tubes. My choice was the mk3. But both the VT100 mk2 and mk3 are great amps and pretty much a bargain at the price the used ones are going for.
Nothing "wrong" with the MK-I. A very good amp.
Owned the I & III.
It seems like VT-100's can be found very readily on the used market. Do they have a reputation for having reliability problems?
Pdreher: No. But replacing the tubes is a much more complicated procedure than with most other tube amps. One of the after market tube suppliers (I believe it is Triode) has a video of the procedure posted.
One consideration is the tube biasing procedure. The MK III is quite straight forward with only one simple adjustment for each pair of output tubes. The MK II is much more complex and takes quite a bit more time as the process is iterative.
I'm 99% sure that upgrades are no longer available. I called ARC and they said so.
I've owned a VT100 for 10 years and it was 100% reliable, but now needs new tubes and maybe some board work. That is probably why so many are for sale used. When it comes tube-time, you may think about getting a new amp. But it was awesome for the time.
Im considering the VS115, it looks like the same basic amp but with an "open" layout, but same tubes.