Welcome aboard!. Don’t give up, you will get it right, and be glad you did, it’s worth the work.
1st: your preamp. It comes from the factory setup for MM cartridges, it needs to be re-configured for your cartridge’s 0.5mv signal strength and it’s 30 ohm coil impedance. http://www.musicalsurroundings.com/downloads/products/p2-manual.pdf
gain, chart page 4: 40db from factory for MM, top line 1-8 all off.
try 56db gain, 1 in, 2,3,4 off
load, also preset for MM. 1-7 off, 8 on
try top line, 30 ohm setting, 2,3,4,5,6 on; 7,8 off
Gotta change these settings for both l and r channels, it is a dual mono design.
Now how is it?
If problem persists: it’s something else to do with your TT/cartridge’s physical setup.
IF it is just the VTA, you can adjust it easily enough, your manual, step 11, pages 10,11https://www.project-audio.com/wp-content/uploads/2018/03/TheClassic_SB_Superpack-Manual.pdf
Still a problem?
Any TT only beats digital when it is very carefully and correctly set up. There is a lot involved/some tools needed, you received alignment protractor and stylus gauge in the box.
level platter; Alignment of arm; cartridge: overhang distance/two null points alignment/tracking force/anti-skate/azimuth/VTA (vertical tracking angle, which is adjusted by raising and lowering the arm post. rock steady platform to start, if isolation needed, solve that independently.
Factory, store, a friend on site, yourself, someone needs to verify, after it is in place, that all is correct. You will benefit forever by acquiring the needed low cost tools, and skills yourself.
Do you have the same music, CD and LP? You mention streaming, I use factory CD, factory LP for comparisons. CD version omits any TT issues: Thus amount/sound of bass, mids, highs are correct, to compare TT to.
I use this excellently recorded music to verify system l/r balance (both l/r volume and lack of any frequency shifting l/r) then compare CD/LP after I carefully set up my TThttps://www.discogs.com/Al-Di-Meola-John-McLaughlin-Paco-De-Lucia-Friday-Night-In-San-Francisco/mast...
Side 2, tracks 2 and 3, all 3 guitarists play. Not only CD to LP comparison, these 2 tracks are a big help with final anti-skate adjustment after all else is correct, listen for distinct l/r guitars and audience vol l/r. more or less anti-skate will effect both l/r and it will make the center guitar less distinct, perhaps a bit off center l or r.
anti-skate: start with what the manual says, but use known music and your ears to refine.
CD version, proves your system is balanced, you will here all 3 guitarists distinctly, strong center, very distinct l and r. Good? Great!
Now, LP version. Hear the sibilants/weak bass problem still? Balance, imaging, as distinct as the CD version? These 2 tracks reveal when it is wrong, and wonderfully when it is right!
I can tell you, my LP version, on my very carefully setup TT (i’m 72, lots of setup experience and all low cost needed tools) beats the CD, certainly no sibilants/weak bass.
Enough Bass: I’m interested in the subtleties of Jazz Bass and drums more than canons, so I use these two: https://www.discogs.com/Niels-Henning-%C3%98rsted-Pedersen-Sam-Jones-Double-Bass/release/1703343 https://www.discogs.com/Mickey-Hart-Planet-Drum/master/246917
mids, I go to Cassandra Wilson’s voice (another to get both CD/LP versions)https://www.discogs.com/Cassandra-Wilson-Blue-Light-Til-Dawn/master/40901
Eurythmics, Sade, Kodo Drummers, and the damn 1812 canons.