i wish i have heard ma900 so that i can try to accurately describe the differences.
mdr7 does not have big peaks like dt880. it's neutral-bright.it's 'neutral' not 'natural'. HD600 is dark with slight warmth tilt. the important question is, do you like neutral-bright sound? if your library are audiophile genre with warm or neutral recording, msr7 is absolutely perfect for that. I'm confident to even say that it is the best in the price range for that. Only PM3 will offer an upgrade. If your library is like mine with modern treble happy recording, you wont appreciate msr7. MDR1A sounds good with everything whereas MSR7 is much more unforgiving. It can sound absolutely amazing or might annoy you when recording is not smooth.
I just realised you had srh 440. I have not listened to them for a long time, so take my words with a grain of salt. Srh440 is very similar to MSR7 in treble levels. If you like Srh440, you'll like MSR7. I do have good memory of SRH840 and MSR7 is slightly brighter than 840.