Not sure what you are really asking here. 'Help me fatten my tone', but then 'the top end sounds thin'. Those two descriptions mean quite different things.
Can you clarify... do you need more fat, punchy low mids, or more sparkle up top. That rig can easily provide both, but those are two very different things with very different solutions.
The good news is, either way, no money needs to be spent!
I guess, in general, I would first make sure you understand the somewhat complex tone stack of the TH500. Start your tone quest with the 'drive' off (the drive is NOT an overdrive knob, but a rather complex EQ/Gain knob), set the gain control relatively low (the gain control IS an overdrive control), and to start, set the mids flat (make sure you understand with the frequency and level knobs do), and slightly boost the bass and treble knobs (both to 1 o'clock to start). That will give you a relatively even, clean, relatively neutral 'start point' in order to figure out what is going on.