I've got a (not very big) problem with recordnig music via USB. I use only clean sound, typical settings for my guitar equipped with 2x Wilkinson WVC humbuckers are:
preamp - DSL, gain - 2.0 (increasing gain over 2.5 makes sound NOT as clean, as I want it to be), volume - 10, B/M/T - 7/10/5, noise gate - 0.5
power amp - American, presence - 2, resonance - 6
I tried Audacity and Presonus Studio One, and in both cases recording level is about -20dB. Of course I can normalize volume in post processing to let's say -3dB and I still have quite satisfactory quality, but I loose this 17dB of signal/noise ratio.
Any suggestion how to increase USB output level?
With those exact same settings of yours I get around -15dB max. (this with high output ceramic humbuckers). Unfortunately: the Preset Volume, as you've no doubt gathered, controls the USB output level and given that you're already on 10 then that's it I'm afraid i.e. it's as good as it gets.
The above being said though: if you're paying any attention at all to gain staging then you're not too far off the limit to be honest (assuming -18dB RMS as your headroom reference level).
If you really need to increase your recording input levels then you could use the line out (headphone jack) into your sound card. In this scenario: the Master Volume then controls the output level from the amp. and it's plenty enough (and believe it or not: there is a difference in tone too i.e, much fuller and rounded in my opinon).
I don't record using either method above to be honest (well: may use the line out method for a quick something now and then). I mic. my amps. up. Cannot be beat really i.e. you get that real Marshall sound (there's no substitute for the speakers themselves moving air I'm afraid).
Anyways. Hope this helps. It really is only because of your particular choice of settings though (obviously) i.e. with "the usual suspects" (distortion and the rest) there's ample output from the USB port of course.