High end sound

I have owned the Magnepan LRS+ and the Mini in 2 small rooms. I found them to be one of the few speakers that would work in these rooms because placement was easier as long as you gave it at least 3 feet from the front wall to the speaker. The first reflection on the Maggie was much easier to tame than say with a KEF LS50 Meta.

I have owned the Bryston 4B-SST before and it should be good with the Maggies, especially with a tube preamp. Bryston had an upgrade program for all old amps. It was a way for someone with a model like the SST to be converted to the Cube lineup. I just checked their website, and I do not see the form to start the upgrade. I would check with them if they still have it.

Service and Warranty – Bryston

I had Monster cables speaker before, and I realized I could do much better. On Maggies I tried a few cables from the very low-cost Blue Jean Cables to the Audience FrontRow (which I think are the best speaker cables I have). My best cables on the Maggies are the expensive FrontRow however, I use the much cheaper Blue Jean cables ($200) instead because they are almost as good on Maggies. Try the 10 AWG speaker cable from Blue Jean Cables. I came across these after a lot of online research from Maggie owners.

Speaker Cable at Blue Jeans Cable

For interconnects I use Benchmark cables. Whenever I need cables, I do not think of anyone else. Cables make a difference with many components. The BM cables give me excellent sound for a very nice price. Your Bryston should not need expensive consumer XLR cables. Your source gear and preamp may benefit from expensive consumer XLR cables. These would be the last thing I would look at.

Cables – Benchmark Media Systems

1) Check placement of speakers (distance to front wall) and reflections

2) Better speaker cables (10 AWG speaker cable)

3) Check with Bryston on the amp tuneup

4) Maybe investigate interconnects

My condolences for your loss. 

My room is pretty well the same size as yours 14.5 x 20 x 9.5 ceiling. It's funny because this past week I started playing around with speaker repositioning and was utterly amazed what simple tweaks actually did for the better.  Lowering the gain on my amplifier also fine tuned the sound making the soundstage wider vs forward, and more holographic with defined imagining. I also experimenting with USB, speaker jumpers and power cords. While my new custom Furutech power cord that is replacing the factory cord for my Innuos Pulsar Streamer has made improvements, my older Ansuz X2 USB sounds better than the newer Audience FrontRow USB, and my Klipsch LaScala factory jumpers also sound better than the Furutech JumperFlux-S jumpers.  Lot's of trial and error! Room treatment is next on my list.  There's a lot of great info on room treatment on Audiogon.  

Take exelent cables from Ice age audio USA -  hi end cables for reasonable price !

Sorry about your wife.  

Cables can indeed make a difference, but it involves a lot of trial and error to find what you like. 

Have you considered room treatment? 

Servicing the amps seems prudent too at some point 

There are also some good mods and upgrades available for the crossovers of your Maggies. Some not only improve sound quality, but make them easier to drive. 

Sorry about your loss.

It sounds to me that your main input is turntable?  If so, can you hook up another source, as an experiment?

You most likely have a cell phone. Spotify lets your stream music at CD quality. Getting the spotify signal into your stereo, could be tricky.  You can hook up a PC with the output from a sound card to your stereo. Maybe your PC has a CD drive?

A pawn shop type of outlet lets will sell a Blu Ray player or CD player for less than $100. Blu Ray and DVD players will play CD's. Try one of those as a source, to see if the sound quality changes, for the better?

Just throwing ideas out there.  See if a different source makes a difference. If the sound stays the same, it points towards the amp / speaker combo.