Best return for the money, system upgrade suggestions please.

I recently added a second subwoofer.
A second M&K Mark 2       8” sub.    Discover Deep Base
I had one originally I purchased 17 yrs ago  had it wired to a separate 5.1 theatre.
Found another one on eBay $160 delivered ,swapped out a 10” from
my two channel set up. Swapped in the two 8” m&k.
Sounded good w one  10 “sub,  But fantastic with  the two M&K 8”.
Deeper Tight Base immediately noticed.
Midrange seemed much better also.
Best low  dollar, night / day difference low cost I’ve ever done.
Something about those original m&k subs makes them very good design
The  MK  8” subs replaced a much more expensive  10” subwoofer previously on the 2 channel ,10”,now in the 5.1 system

System details are in my virtual profile

R

Pay down those loans!

After that, source first. IMHO your system is unbalanced by the middling digital sources, YMMV.

I like Michael Green’s suggestion.

A streamer plus a Tidal subscription is another good option.  So much music to explore for such a small investment.

Room acoustics can cost from $0.00 to $100k and up. I suggest zero for now, there is a lot that can be done with books, plants, moving furniture, curtains, rugs and carpets. I am getting tired of this stupid self-advertising by some.
I would upgrade speaker cables first and get couple of decent power cords, not necessarily spend all the amount you have. This would complete the amp/speakers/speaker cables unit for the time being..