The thing for me is - I found the AI500 rather thick and clogged sounding, as if a blanket had been thrown over the performers. There's no way a solid state amp will mimic this unless it's very sick indeed...
How about a Puresound A30 or, if he can stretch to it, a Croft Micro Basic with Series 7 power amp? I know the latter very well now and it is so good at what it does. Some tube-rolling might mellow it down a bit too (Mullard Long Plates perhaps?) to take the edge off the speakers.
For good used amps, Jimmy Hughes used DNM's for years (probably still does) and these drove the Impulse speakers he had to very high levels with no strain or stress at all. The sonic clarity on a good day could be breath-taking too.
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me