Audio Research VT100 - MKI or MKII or MKIII?

It seems that my prior post regarding this topic has vanished. So I am posting again.

Can anyone tell me the differences between the VT100 MkI, II or III? (I know the choice of tubes makes a huge difference, but I'm asking generically about the amp itself).

Recently, a close friend loaned me his VT100 MkI, as he is not using it. Damm if this thing didn't shock me. I mean it sounds really, really good! Now for the delima.

He wants to sell it (asking $1500..all original tubes, so I assume I'll have to replace them fairly soon, but otherwise in excellant condition), but I've seen MkII's around $2000. Is the MkII a step up from the MkI? I know I can change power cords with the MkII (a feature that I like), and that it has double the power reserve of the MkI (which usually equates to better dynamics and punch in the bottom).

So, what about the MkIII? I notice it lost it's tubes on the input stage, but still uses tubes in the driver stage. Is this a step up sonically?

I'll be driving a pair of Von Schweikert VR4 Gen III, SE's in a 15x19 room with 8 ft. ceilings that are coffered.
So how 'bout it fellow A'goners? Should I spend the $1500 on the MkI (excellent condition), or should I spend a little more on a MkII or even a MkIII?

Oh, two last things. First, I'm a tonality freak, and I've heard (through the grapevine) that the MkIII is not as warm or as tube sounding as the MkI or the MkII. Is that true? Secondly, I insist on the bottom end being full and rich. The MkI fills the bill here, but could use a little more transient attack (ie, punch--maybe the MkII?).

Please give me some thoughts, as I have no way of hearing any of the other VT100's. In advance, thanks a million, and I look forward to hearing back from as many of you as possible.

Merry Christmas, Happy New Year to all and above all HAPPY LISTENING!!
If you send the MK I to ARC for retube, you are looking at about $800.00 plus freight. Find a Mark II that has been recently retubed and call it a day.
In addition to lg40's recommendation, I would look for NOS replacement tubes for the input stage such as Siemen's 6922's. This will smooth out the tonality quite a bit from the Sovtec's.
MKII upgrade was a leap forward in every way, especially in the bass! I've not heard a III. I run mine on a PS Audio P600, with the fan on a seperate circuit so the PS multiwave doesn't screw the cooling. That adds even more and tighter bass.