Actually, I'm not a modder or a kitter. My amp is a Heathkit because that's the name of the company and they made very high quality kits. But I bought mine used, which means it was already assembled. I agree about how--if you're handy with a soldering iron--you can upgrade the parts in a signal path to great effect. I've never done any modding except for some tube-rolling.
12-09-11: Orpheus10
If you're into "kits", you can also get deep into the "high end" by substituting parts. For example you could substitute better capacitors, of the same value of course; and other parts as well. This would give you a true "high end" amp.
I have a Jolida JD-9A phono stage and there are slews of mods out there including capacitor upgrades, but I never learned solder well. However, there's a lot of buzz about replacing the factory socketed op amps (about $1.19 ea) with $18-22 ones made to greater speed and lower noise specs. I'll probably get a chip puller and pop in the upgrade replacements.