moding and the inexperienced

Unless you are talking about the GFP-750, be wary of expensive upgrades to Adcom preamps. You'll quickly find yourself throwing money at products with limited performance potential and poor resale value. "Reasonably priced" and cottage industry audio modifications don't co-exist in the same universe. If someone has the skills to experiment along these lines without incurring labor charges, fine, go for it. But if you are paying someone to do these mods to a mid or lower level adcom product, you might as well pour money down a hole IMO.

If you do not own a piece of equipment why buy one you have to modify? Usually modified equipment (unless by very well known experts) is worth less, not more than stock items on he used market and one can rarely recover the price of the mods. I suggest that you read a couple of books on circuit design before making any decision. The VTL book and Valve Amplifiers by Morgan Jones are geared to beginners.

I have to agree with Photon , and I will add , unless you have a friend who has had the same mods done to the same piece of equipment , and can give you a first hand view of what to expect , your probably wasting your money .
Some of my friends have had higher end gear upgraded with good results , but it took lots of research and money to get good results and sometimes the gains were marginal at best .