So I just recently purchased a used DSL5C online without trying one first (no decent music shops within 100 miles...) and I'm not super stoked on how it sounds. Wondering if there are any good options of upgrading it? Different tubes, speaker, etc.

My main issue is with the Ultra Gain channel, sounds kind of weak and overly muddy. I am running a Gibson Dirty Fingers bridge humbucker so that probably has something to do with it. Maybe a nicer instrument cable as well? Trying to not dump a bunch of money into this thing as finances are pretty tight (Damn you Xmas!!).

Maybe there is a different amp that I should have looked at also? Open to suggestions, although I definitely need a headphone jack and something I can play at low volumes. Also footswitchable channels is a must!

Thanks for your help!
the first step to failure is trying.
You can try either the celestion 10" Vintage 30 or the Celstion 10" Greenback. either will set you back at least $100.

you could try some new preamp tubes. The new Mullard CV4004 is a very nice tube, so is the JJecc83
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
I don't have any experience with the DSL5 but for 10" speakers look at WGS. They have a Vintage 30 type and a Greenback type for $69.

As for other amps if you need headphone level of quiet a tube amp is not your friend. For headphone practice I use Amp Sims on the computer with Peavey ReValver and a Rocksmith Instrument to USB cable and good monitoring headphones. The headphones I have cost more than the software and interface combined.

Or use something like a POD with the headphones.
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .