Okay, I can make this fast... (plus I am rushing out for the evening...)
I just did another session with the M + Lab 1, with and without the AClear Porta NXT-2AK. Well... mostly with. To my ear, the M + Lab 1 sounds so much better when you add the AP to the chain. Whereas the AClear Porta cuts of the testes of the M + Heaven VI, it absolutely brings the Lab 1 to life. It is subtle in some ways yet really meaningful. Tonally there is no change - which is good. The bass is about as strong, just less bloom and better controlled. Everything from highs to lows is clearer, more defined and more lively. The wonderful layering of the Lab 1, already a fine match to the M, is simply enhanced.
Don't get me wrong, M + Lab 1 is a wonderful portable rig, almost an end-game portable rig. Until you add the AClear Porta into the chain. Now I cannot go back.
So for me it is:
M + Heaven VI FTW
M + AClear Porta NXT-2AK + Lab 1 FTW
M+ AClear Porta NXT-2AK + anything else that used to distort from the M FTW
All my tastes, of course.
My guess is the impedance matching.
Lab 1 is 8 ohms while VI is 16 ohms.