Audio Research VT100 Mk1, Mk3; VS115

I have a VT100 Mk1 and seek  to upgrade mainly due fo age.  (Speakers are Watt Puppy 7's.) I'm not seeking a big improvement but I certainly don't want to go backwards.  In another thread  people stated that the VS100 was a step down from the VT100 Mk3.  One person stated the VS115 was better but was essentially the VT100 Mk3 on an open chassis. What I didn't hear was what the difference is between the VT100Mk1 and Mk3.  The VT115 was said to be solid state sounding which is not what I'm looking for.  Perhaps I should just stay with the Mk1 and be prepared to fix it when it breaks down.