Brian, I'm no acoustics expert. You'll get all the advice you need on that from others here thankfully. I am just starting to get in a similar situation as you are at now. I just received 4 Mini traps from RT for bass trapping some ceiling/wall boundaries. Right now they are just on the floor straddling corners and I will hopefully install them up top this weekend. This is not an ideal amount of coverage for my room. I should have about 8 traps, but the results with 4 traps are very promising. I didn't have what I would call one note bass in my room so it wasn't a dire straits situation, but there is no doubt I'm getting flatter bass response in the room with these. I will be getting either one or two pairs of the GIK Tri traps in the next couple of weeks for floor/wall corner trapping which will get me up in to that zone of 8 bass traps for my room. After that, I'll look at reflection treatments like you are now.
I have one suggestion for you though. I believe you stated your rack is on your front wall between the Tri traps and speakers. And you have a turntable too, which I think makes this suggestion even more worth considering. And if you have a wood suspended floor, again more worth considering. Move your rack to a side wall and run some longer speaker cables. I did this after reading Jim Smith's book, Get Better Sound. And he did make this recommendation in strong wording. I was skeptical about the results, but very impressed once I did it. Jim's book didn't really state why this is better. My theory is it has to do with acoustic reflections off the rack that negatively impacts imaging, but I also wondering if it has to do with mechanical resonance from the speakers affecting equipment in the rack more so in between speakers compared to when on a side wall. And I believe a rack will work better the closer it is to a load bearing wall, especially if its a suspended floor.
If this is something you can consider, and need some affordable, longer speaker cables to try it, I recommend Anti Cables. I got a pair made up where the lengths are different, one shorter and one longer, and it works great. The longer pair was twisted to give it similar characteristics of the shorter pair. And there is some other technical aspect of the cables that makes the suitable for having different lengths. And the cost is low.
Bryan