Hi Sharkygolf,
Both the VTL 2.5 & 5.5 preamps are excellent pieces, with superb sound, reliable & well build with very good resale value. The VTL 5.5 performs at a higher level, but at additional cost.
I have the VTL 2.5 & enjoy it immensely in combination with a Plinius 8200P. I would consider the 2.5 an appropriate match for the ST85 on both a cost & performance basis.
However if your plans are to upgrade your power amp in the future, then perhaps the 5.5 now is your best option. But if money is tight, then the 2.5 is probably your best choice.
Either way you'll surely enjoy sonic bliss.
Regards.