Speaker upgrade for classical music

Long thread. I'm going to depart from everything said here and say that the one thing that you said you don't have is actually the thing you should have. If you are classical musician and really want the sound of a symphony you need to turn to vinyl. No digital or CD signal will ever give you that experience that you seek no matter how much you spend on equipment. Vinyl pressings from the '50s '60s and early '70s were far better than anything made today except for rare exceptions. The fabulous thing is that nobody wants vinyl classical and it can be had at ridiculous prices all the time.

You need to look into this seriously and, when you do, look at classical recordings on London vinyl which is actually English Decca made for the US market, EMI, Angel (only those pressed in England, Phillips pressed in Holland, and RCA Living Stereo Red Seal with Nipper (the RCA dog) in the shade. You will spend a fortune trying to accomplish this with digital media. There will be people who will disagree vehemently with this statement. If you love and know symphonic sound vinyl and tape are only way to get it. 

There are exceptional CDs and digital streams available of truly great performances so there is no question that there is great music in digital format but if you're really looking for the true experience you will save yourself a lot of time and money by getting a good turntable, an exceptional cartridge and a world-class stylus. Also you will need a good phono stage. 

Follow some of the value-oriented advice that you've gotten here on speakers so that you don't have to spend all your money on them and now have enough to pursue vinyl and start building an incredible collection for next to nothing.

Scansonic new editions or marten speakers 

As a listener who enjoys large scale music, including classical, my recommendation is to find a pair of Emerald Physics 3.4s or 2.8s. The 2.8s are 3.4 PLUS dual 15" carbon fiber woofers per side. Both are 93dB+ and easy loads to drive. Currently running my 3.4s + 2 SVS subs

Since these are now discontinued they are available used under $2K

FWIW I had Magnepan 3.5Rs prior and never could get a proper seamless blend with my subs, even tried biamp and active XO, and anybody's guess how well your amp would drive them

hth

@ssmaudio  To actually answer your question, the bass issue would depend on your room and where you decide to position (or have to position) the speakers.  Also, remember that larger speakers that need some kind of stand to lift them a little off the floor can have a lot more internal volume than many small floorstanders.  I suppose technically the LS 5/8 is a monitor, but it doesn't conform to our usual modern understanding of that term.  The REL's you have are good subs, so to my mind this is a non-issue.  Get the best sounding speaker you can, then worry about lower-end bass later.

BTW, that recommendation for QLN a few posts ago is an interesting one.  They were very high on my "try to hear" list, and I have every reason to believe they would be very good.
https://parttimeaudiophile.com/2020/05/15/qln-prestige-three-loudspeaker-review/

Another Magnepan recommendation here. My friend who played in an orchestra and has an amazing system of his own always loved hearing my 1.7i now upgraded to 3.7i and would say it was the most realistic sound he ever heard. I'm using these in a 12 x 15 room.
Hopefully the pandemic will be over soon so you can hear for yourself.