Hi Peter:
Did you buy the BA-5000 you had mentioned? As for another one so that you could run them in 600 W mono, there is one on Ebay now:
http://cgi.ebay.com/SANSUI-BA-5000-SUPER-P...%3A1%7C294%3A50I seem to recall this one was on Ebay before, and its missing the right handle (and looks like power switch is non-original either), so its up to you to decide how much you want it, or if you prefer to wait for another one in better condition. Not having a handle is a drawback since this amp is VERY heavy, and trying to lift it without aid of a handle is really asking to give yourself a hernia.
As for doing any mods, it would be best to have someone in your area do it, and minimize the time spent to have to ship it too far, considering its hefty weight. With that in mind, two people I can think of are 'Quad Bob' (well known online, great guy, very knowledgeable) and another person I know is Rick at Allegro Sound - Ive spoken to him on the phone - he is a bit of a character with very strong opinions, but he seems to have alot of experience with BA-5K mods, so you can always contact him if you're interested:
http://www.allegrosound.com/B/F
ps: Hello Jim- Thanks for the kind words, you've always been a good Sansui friend of mine. Stop by sansui.us sometime, you'll find me there on the forums when time allows. I just added another amp to my collection, a BA-F1. EXCELLENT amp!