Very, very different. First, the DB750/1 is really a boat anchor... super heavy, three rack spaces.
M9... super deep bass extension... massive low end and IMO enough sub bass that an external hi pass filter is almost needed with some cabs. Very little 'tube complexity' in the midrange... very solid state sounding. Very bright and aggressive up top (which can be controlled with the graphic pretty well. Probably the highest output head out there, regardless of wattage. Reliability VERY spotty (lots of shutdown issues... so careful buying used, but they seem to have it fixed, and service and support are very good).
DB751.... louder than most would ever need, more controlled low end (not as deep, even with the low boost engaged), nice midrange complexity, relaxed up top. Really different. Limited EQ power (although better than the DB750)... can be a bit picky with cabs due to that. Nice warm, tube complex midrange. Very good service and support.
Fat low end, warm mids, relaxed top end.... DB751 is very nice. Aggressive grindy top end with massive low end kick, hard to beat the M9.
If you are not playing auditoriums with no front of house support and/or don't need 2ohm operation and/or aren't driving two 410's or whatever, the Aguilar TH500 would make most DB751 users pretty happy, and the GK MBF800/MB800 would make most M9 users pretty happy at a fraction of the size, weight and cost.
These amps would IMO appeal to totally different players.