Not a bad amp at all. K-ON songs do range from simple to hard rock, so it all depends on the songs you want to play.
For a song where guitar takes a back seat like
Prologue, lay back on the gain and let the mid levels ring higher than your treble dial is set at. You can play around with the bass knob until you are happy, but the preset knob could probably sit on anything on the left side: twang, blues, or class A. Clean would have almost no kick to the sound so I'd leave that out for now.
For a song where guitar is up and in your face like
Gitah ni Kubittake, crank that sucker onto the right side of the presets and slowly adjust the gain up and down until you get what you like. Don't adjust anything too quickly on an amp, there's always the risk of blowing it straight out of the water and having it overload- I'm not quite sure just how loud you play. One mistake lots of people make is lowering the bass and raising the treble a lot. It sounds heavier, but the second you take into rhythms and chords, it has no backbone to the tone. A general good tip I think I found from experience is to have the bass set to about 6 or 7, having the mid set to 5 and the treble to about 4. If the bass feels to heavy, flip the treble and bass settings- some amps are made more for metal where the bass has to be nice and powerful.
Then there are the songs with wah-wah and other effects like in
Joyful Todays. For this, you'll definitely want a stomp pedal or an effects board. These do get pretty pricey, so maybe think about the latter choice as a one-time investment; they are extremely useful. Since K-ON is generally pop-rock to hard rock at best, perhaps the amp you have isn't optimal for playing K-ON specific songs, but it'll definitely work as is. The Spider IV 75-watt has presets from bands like Slipknot and Lacuna Coil I believe, so it was probably built for heavier forms of music. Like I said though, it definitely does work and it has a clean side to it as well, so the best way is to play around and find what works best for you. Every ear has a different perception of what sounds right and wrong, so as long as you remember the basics like tuning and tonality (making sure you don't have feedback, additional sounds, proper clean sounds), then everything else should tune in naturally over time.
Good luck.
