> if i could get away with removing the BJTs and using a simple resistor network and opamp in inverting mode.
That's just the mix-amp. To study a mixer, we need to know the the size and number of mixing resistors and how they are switched/panned. These are typically on the channel card or assign switch.
With a glance and a guess: do you have ANY trouble with hiss noise? I suspect not. There is no gain between mixamp out and the fader, only 12dB gain after the fader. If you don't keep the fader very high, "normal" level at the main outputs could be very high level through the mixamp and the fader driver, which could be your "mushy". (Especially if the main faders are <10K and driven by TL084 chips.)
The mixamp feedback resistor is awful low value, 1K5. If we assume the channel mix resistors are also 1K5, resistor noise will be quite low. However channel-amp load will be 1K5 for one bus, 400Ω if the channel drives four buses (equally, without pan). A 5532 won't suck bad, but a '084 will be in great distress driving high channel levels in 400Ω.
Also the very low mix impedance means there will be great honking chunks of Class AB output current half-waves flooding the supply rails and thus the ground system. No special care is shown for the mixamp ground reference. The mix could be standing on a ton of garbage. This was pointed out years ago, by Doug Self and others: lowering the mix impedance lowers thermal noise but at some point (with multiple real inputs) ground-garbage dominates thermal noise. Solutions include transformers and differential mixing; MAJOR cost increase. Patches include GOOD local decoupling near every hard-worked chip, in the channels and in the mixamp. 0.1uFd per 5532 and nothing on the '084s is probably less than you want. I'd think more like a good 0.1uFd ceramic next to every chip, 10uFd within 2 inches, and 50-200uFd per board (either a lot of 10uFd or a bulk 100uFd in addition to local 10uFds).
Noise: 12 or 16 channels at 1K5 each is a 100Ω bus impedance. Thermal noise will be 0.2uV. Chip op-amp noise will be 0.5uV-2uV. So the transistors, working much richer than the 5532 inputs can afford to go for general use, will give a lower noise floor. At least with all channels un-muted but faded to zero. In any real mix, source noise should probably overwhelm mixamp noise.
So the BJTs are for test-spec. They may have little effect on noise in real use. Noise will be 114dB below +4dBm and 130dB below 0Dbfs. You need 21-bit recording before that is a problem. And 130dB mixes: there ain't no such thing. I've faced 120dB SPL peaks above 14dB SPL room noise, and that was an absurd situation (I made no attempt to capture the 106dB dynamic range, since I had 95dB recording and the judges would be in ~80dB S/N listening rooms.)
If the channel resistors are 1K5, then for 15 inputs the mixamp runs at noise gain of 15. If the mixamp were a low-spec naked '084, 3MHz GBW, gain is flat to 200KHz and we only have 20dB of NFB at 20KHz. For a stage that must handle high levels (because there is little gain to the main outputs), that's really not enough for a complex mix. The BJTs are biased/loaded for a gain of about 15, so they add to the 5532's gain. The combo gives gain of 15 at 6MHz, the naked 5532's unity-gain point. So we have 50dB NFB at 20KHz, which is enough to clean-up any decent audio chip. Note though that if we muted channels by floating their resistors, the 15:1 mix-loss would vanish and the opamp would have to be unity gain stable. This BJT-5532 combo is NOT unity gain stable, and it would take cleverness to make it so. (Actually there is some there already: the 100Ω + 0.03uFd termination. It might actually be stable with all channels floated.) The disadvantage of requiring all mix resistors connected (to channel or ground) at all times is that you always run at maximum noise; floating unused inputs reduces noise gain.
Before you go too crazy: try running your Masters at max and reduce all input trimmers to get back to proper output level. If it is less mushy, you have been overdriving the mixamp and fader driver.