Advise on Mcintosh mc40


Hello,
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
128x128tubesmore
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.
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 - http://classictubeaudio.com/
Thanks for all the advise! one other question is what tubes can i use as a subsitute for the 6l6?