There is really no such thing as a flat eq, as a lot depends on the cabinet and the bass you are using, even the strings, so tweeking the 750 could work.
The thing I like about this amp is that set flat it is just so full but with no mud or boom.
I think that's because of the high pass filter that keeps things sounding tight. I think the 750 can get that Staccato tone pretty closely, much moreso than the 450, thanks to the Tweetertone knob. The 450 wasn't able to dial in that crispness up top, but the 750 goes there no problem.
I did find that it compresses the low end significantly over half volume which I understand is to make it more valve like. I would like to retain a fuller sound for longer.
Do you find there is significantly more headroom before the low end starts to compress on the 750 over the 450?
I never used the 450 at max volume really, so it's hard to compare to the 750 in this regard. That said, in the bedroom, side by side, the 750 sounds bigger, fuller, louder than the 450 even at the same EQ settings. With just a single ML112 though, it's hard to compare max output. Both amps are just plain loud, period. What I readily noticeable though is the "more willing" response of the RH750 at would I would call loud gigging volume, which is not really pushing either amp.
My theory on this is based on a couple things, first that both heads have the same continuous output that is controlled by the APM. However, the 450's burst rating is the same as it's continuous output rating. The 750's APM is different in that it allows more bursting power from the power section, which also has more power than the 450 has. All that translates to me into a bigger, punchier head.
I've not published my own scope on the 750, but I have taken lots of measurements. The Tweetertone really extends the top end on up, even boosting the super highs above 10k as you dial the knob up. In other words, it not only flattens out the top end, but it also boosts it into Marcus Miller territory. It sounds great with the ML112 since that cab of course eliminates that response with the mid driver. I like that Tweetertone knob as it is KILLER with the IEM stuff I do to control zing in my ears, which can ruin a gig for everyone. I also like the placement of the compression knob over the 450/Staccato.
So, those are the reasons I think the 750 is a better Staccato. More power, and complete control over the highs. The Staccato is a more finicky head with cabs I think, and could be harder to use with other things down the road versus the 750 which can literally be tuned to whatever cab you want to use it with thanks to the 4 bands of semi-para EQ PLUS a Tweetertone knob. All IMHO of course.