We did our floor ourselves. The floor joist were correctly installed so we just added bulk to the weak floor. 3/4 plywood and 1/2 soundboard.
Don't expect a miracle fix. it did not correct the weak bass in the room or the reflections on the sloped walls but it did make the floor solid. The only way we could get any decent sound was positioning the speakers at the longest end not at the width. The speakers set facing the short sloped wall will create a bouncing effect from the forward wall to the rear sloped wall. Room measurements will show this. We treated the slopes and the ceilings but the " look" was odd not to mention the headroom was reduced.
Mike hope whatever you try works out.
I agree with you Chris; in Mike's situation (and mine too by the way, but not to the same extent as my ceiling is 11 feet high at the center and 8.25 feet high at both sides, plus the system is set along the long wall, just like Mike did this morning, but I have five feet behind my speakers to the front wall, and five feet behind my ears to the rear wall) solidifying the floor is not all ends to the final solution.
{I would have prefer the short wall but I compromised for other decor/furniture practical related priorities.}
And I did mention contacting a professional acoustician, among other suggestions that he can try but for free (proper positioning of the speakers and listener, based on solid acoustical/mathematical foundations - proven science).
And that, I experimented with (fractions of 1/3rd, 1/5th, 1/7th, 1/9th, 1/11th and their multiples).
* One sloping wall is manageable, two as well but with more astuteness's requirement.
And what MR JAZZ (and myself) also suggested is bass DSP EQ (parametric or FIR or IIR filters). ...And a pro acoustician can also help in the mid-high frequencies.
If Mike is very serious about all this, and I believe he's pretty relaxed and simply more curious right now at the easiest solutions (like alternate pair of loudspeakers for that room, and not dealing with major room's modification), he might contact a competent acoustician for an assessment of his room, and work with him on the possible solutions with the less stress possible (and expense as well), and some good old fashion and judicious room treatments mixed with smart digital EQ.
We're just talking.
P.S. In Mike's room I would use the short wall (main window above his garage); just like he did from the very beginning.