Advise on Mcintosh mc40

I just got a pair of mc40 monos, They sound Great but I was wondering what upgrades if any would be best. One of my main concerns is that i dont want to lower the value which brings me to another concern what are they worth. they were part of a package deal i made that also included a mcintosh mx-114
Thanks in advance for you advice
GaryJ Vogel
Thw standard repair advice would be to ensure it meets spec . Then to replace the all of the old electrolytic and the other caps as needed.
The value of an amp is only estimible for a moment in time, and for an amp of a certain condition. The best way I think to do this, is to watch ebay auctions through to the final selling price....not what is asked.... but what it selles for. If you follow several auctions you will get an idea of what people are willing to pay for that amp in a variety of conditions. What somebody will pay is what it is worth.
The AudioGon Blue Book list the average price at $620.00 each, check the closed items on eBay and maybe do a national Craigslist search. I would have the amps checked out and re-capped by a good tech, like Pat at Classic Tube Audio -
It will also depend on who does the modifications/upgrades. If you use one of the well regarded McIntosh restoration companies(Terry DeWick as an example) , it could actually enhance the value down the road.
Thanks for all the advise! one other question is what tubes can i use as a subsitute for the 6l6?