I'm late to this party, but a few thoughts...
First, "ktm125," huh? Wear earplugs!
All the comments about taking measurements & addressing acoustic issues are good advice.
If your Focus SE speakers have an impedance curve similar to Stereophile's 20/20 measurement above, the impedance below 100Hz is a concern. At 30Hz, they have 2 ohms at -45*, which is a
serious current hog! I'm not convinced streaming-only demands the very highest-fi amps, but you definitely want an arc welder for that. Perhaps check with
@John_Siau before ordering the AHB.
If he tells you to look elsewhere, I'm not even sure what to suggest. An old Krell? Pro amps rated for 2-ohm speakers? Maybe biamping, just to isolate the bass load... Whatever amp(s) you buy, do NOT bridge them.
(Honestly, after seeing those measurements, I'd at least audition a few speakers before spending $5k on electronics, but that's 100% IMO. I fully understand hanging on to old stuff you love.)
As noted above, you can get smoother bass over a wider listening area with multiple subs and EQ. This would also allow you to highpass your mains, skirting the impedance issue. Seems a shame not to get full bass from those big speakers, but it's definitely worth experimenting with. Before buying subs, you could try a filter in, say, EQ APO to see if midrange clarity improves, dynamics seem tighter, etc. It's hard to judge when you lose 1-2 octaves of bass, but testing it is free.
If you want to experiment with subs, Dayton's $150 12" is the budget choice. Hsu, Rythmik & SVS all have solid models for about 3x the money or more. I'd lean to Rythmik L12s, if I were feeling spendy, but I'd try three Daytons first.