I have had the following amps for a weekend trial in my system: ML-336, Pass Aleph-5, Pass Labs X-350 and BAT VK-500. My recomendation goes to BAT. Nothing could match its natural presentation, timre that was true to live instruments' and (surprise!) best bass slam and control. Pass X-350 was second - due to definitely worse timbral accuracy and clearly worse bass definition and dynamics. ML was third in this rank due to rather cold and analitycal presentaion. Pass Aleph-5 was nice, but not in the same league as far as power and control is concerned.
These amplifiers were tested with Wadia front end and JM Lab Utopia speakers.
What I would suggest is to take a listen to BAT before spending more money for Pass X-series or ML.
Kind regards,