Up untill to this moment, I was exactly in the same situation. yet, in the final moments of musing, I managed to narrow down the competition to just two:

AMB (M^3)

vs.

Everything Else

Why?

Because it is catagorized at this price range of "if it weren't a DIY product it would cost a lot more"

The M^3 is a "DIY", which, when compared to commercial products at the same price- it excels in killing every other opponent who dared confront it, in terms of them soundstage, details, neutrality , "rich sound" , many aspects-you name it...

"Meier Audio" very good amp, but it's twice the price (almost 700), so it's out of the question, right? are we talking budget here ?

if not - why not decide between the SinglePower Extreme and MPX3.. beta22..Audio-GD Reference (one of the the 1-9 series).. there are more of course. 

Purity Audio K.I.C.A.S caliente... Sorry, The Caliente is nowhere close to the M^3 in terms of power or any measure of technical prowess (some think)

I read something funny regarding; on one forum, some guys said that the tube feeling it gives, is close to the tube feeling the "beresford" gives when compared to other SS Amps in it's price class...)

Sheer Audio - it's like a nice KECES, (again, some think) .. I read that "There is higher noise floor of the HA-006+ with ..."  

OK

Now, between the -M^3 versus C2- : this was hard. both have the "DIY" cost/performance ratio , but  

statistically thinking: 

M^3; there are various builders, different configurations, it's more customizable, even scalable!

very easy to trade, less reduced value when selling. more of it appears on forums... i'd say it's "famous".

The M^3 has optional bass tuner... good for different can's (say..from what i know... Denon's ADXXXX/Sennheiser HD650 bass ..  vs AKG's 'bass' ... if you ever switch...) na ?

As for this day: Audio-GD has 6 H/P amps (4+2 musical) , and from all of which, the C2 is the cheapest one in the group. Fact.

AMB's got 3 ...and the M^3 is in the Middle. 

DISCLAIMER:

that's only what went on my head...There are people also here on the forums, which thinks otherwise. every one has his own "cup of tea", right ? 

I DO NOT take any responsibiliy for the aforementioned, whatsoever.

all those companies are respecful, and you should try to hear them all and then decide. YMMV.

-Eyal.