Before commenting on the sound problems you guys are having, I'll try & clear up the Pre-set query. It is very important to read the manual to understand exactly how the amp functions - it isn't like a conventional amp.
Pre-set mode is set at the factory, and each pre-set is intended to provide a 'typical' setting for each amp model. This consists of not only the AMP model at a particular EQ & volume setting, but also selected EFFECT(s). So, when you turn the dial to an amp model, you'll automatically get the factory selected 'typical' effect that goes with it.
The pre-set effect can be by-passed by pressing the BYPASS button (pressing BYPASS again will re-activate the pre-set effect). The amp will 'remember' your BYPASS preference - but nothing else. So, if you select a pre-set, and tweak it to get the tone you want, the amp will revert back to the factory pre-set setting for that amp model every time you select that pre-set.
You only have TWO (CH1 & CH2) places to permanently save a 'custom' patch which is very limited. (Vox has expanded this facility to 8 patch memories on the brand new updated VT range). For this reason, and to avoid the pre-set effect, most ADxxVT owners tend to use the amp in MANUAL mode (WYSIWYG - 'What you see is what you get', on the amp controls) with CH1 & CH2 containing two favourite patches - but not the pre-sets. The reason for this limited design is because the AD15/30VT were only designed as low cost but well featured practice/small club amps, when certain compromises had to be made as between cost & features. Although the AD15/30VT proved successful, spouting the bigger 50/100 versions, the amp design was unchanged.
OK, now turning to your sound quality issues. It's always difficult to comment without hearing the amp, but a common issue with all the Valvetronix amps is the jack sockets. They oxidise easily, and the calipers can get stuck. Clean ALL your jack sockets with electrical switch contact cleaner & push a jack in/out several times (make sure it is safe to use with rubber/plastics - some versions aren't). Dirty sockets will degrade tone, making it muddy and/or crackly - in some cases, you can lose sound altogether. Stuck jack sockets can give similar problems - particularly common is the headphone socket 'sticking'. If your sockets are very oxidised, several cleanings may be necessary.
Another theme is that modelling amps use software. If the software file paths get corrupted, you can have sound quality/functionality issues. A re-set back to factory settings will usually cure this (similar to re-booting your computer).
It is possible that the tube might be defective. Usually, the tube will last the life of the amp (these are not used in the same way as in a conventional valve amp) & it is unusual for these to give problems - but it has been known. Try the first two maintenance tips first, before replacing the tube.
Hope this is helpful.