i have several friends & acquantance that i have met over the years that have owned the Harbeths and none of them uses the same amp.
To witt, here is a short list of amps that I know from different people and friends over the years that are being used with either the M30, SHL5 or C7es series:
1. mcintosh s/s amp
2. Cary 805s
3. Krell 400 int amp
4. classe int amp (even the older version)
5. plinius int & power amps
6. mccormack dna amps
7. Cayin tube int amps (el34s)
8. Accuphase Integrated amps (class A and Class a/b variants)
9. Bat VK 75
10. Luxman 505 int amp
11. Conrad Johnson MV60 & LPS70s
12. Naim Integrated amp
13. LFD Zero LE MKIII
14. Bryston can work but a bit tricky
15. jeff rowland Capri S & 102 power amp
16. Bel Canto Digital amps
17. Peachtree Audio Nova 80w hybrid amp
19. Sun Audio 300bs
20. Pass XA series
21. Ayre V5 power amp
Krell and harbeths can work together if you use a warm sounding cable like cardas or purist audio design.
In other words, they seem to work with just anything out there. Its a matter of preference.
As for me, the absolute best is the one that fits your sound. I love the Luxman int with them, but does not mean to say that the Classe power amp wouldn't work well with them. So, anything goes!!
I would suggest you visit the Harbeth User group as these guys there have different choices of amps they have used with the different Harbeths.
These are easy to please speakers folks!!