In the 14 months I've had the M4 Turbo I've had 4 amps on them. A 2A3 set, a Berning ZOTL 10, a Cherry STM and finally a Berning ZOTL 40. The M4 exposed their weaknesses, with the exception of the '40. The speakers sound more linear, punchy and alive with the '40. It's very quiet/detailed, transparent, focused, extremely dimensional with a very open top end. If it fits within your budget, get one, you won't be sorry. Even Clayton has one.
When I talked to Clayton last week he said the M4 mods should be done in 4 weeks or so.