Having somewhat recently gone through a similar mixer remodeling project there are two things that seemed most important:
PSU upgrade, and decoupling upgrades across the board.
The net gain of improving those two depends on how well the manufacturer implemented them in the first place, but I'm willing to bet even DDA has skimped on these. I actually didn't just upgrade the PSU, but completely replaced everything but the case. decoupling is easily tested channel by channel (or group by group) basis. Those DDA modular PCB's seem sparse, which leaves plenty of room for on-board additions.
Random noise bursts, crackles etc and all mentionable hiss disappeared, with original manufacturer specifications improved remarkably. I used to hear when even ours neighbours fridge turned it self on. The PSU now blocks it.
Since you have a whole 24 channels to experiment with, a very interesting thing to try is opamp upgrade variations. For example, read some of Samuel Groners massive opamp shootout conclusions, select like 5-10 most interesting opamps, and swap them in. Some work really well as filter buffers, others across the board, others only as "interesting" (but maybe usable) distortion generators. Others handle over-driving gracefully, others become downright nasty. Unexpected things will happen, like some channels with cheap opamps generating obvious (but subtle) harmonics and always sounding just slightly louder than other channels on perfectly equal (measured) gain.
This way I ended up with 16 subtly varied channels and it was a fun experiment. 'High end' stereo group here, 'muffled stomp box' there. Maybe you'll end up with a brilliant sounding "coloring book" like I did.
