Audio research M 300 mkII vs Classic 150 triode,

I have to confess, I have a fealty to Audio Research. that having said, I am tormented as which I should purchase for my Martin Logans Monolith II. I listened to Jazz and RB music. I like both amplifiers, however I am told that there are frequencies roll off at the top(high frequency) and bottom ends(low frequency) for amps in the Triode mood?(Classic 150). Is this true. I have about 10 days to complete my purchase. Audion audiophiles have been of tremendous assistance to me.