The best solution would be a raised floor. If you only want to isolate the amp you can get an amp stand and put between the stand and the amp some kind of absorver. Also remember that the frequencies that go through the building are mainly low frequencies due to their wavelenght, so try to keep the Low frequencies in your amp down.
Instead of just selling pianos you could rent them. A piano is an expensive instrument and not everyone can affor to buy one, so you could rent them, in this way you could have a stable income and get future buyers.
You'll have to route the hardware outputs of you aux channel to a pair of outputs in you audio interface. I see that you have a fast track c600, so outputs 1 and 2 are your monitoring outputs so you can chose outputs 3 and 4, connect these to the inputs of your lexicon, the the outputs of the lexicon into the line inputs of the c600.
check out the ibanez rgd, any guitar will work with heavier strings, but if you can live without a tremolo get any fixed bridge guitar and you be good to go. Just watch for some intonation and action problems that may appear due to the heavier strings
Try the pod hd, the engl e530 and the 6505+ 112, they're all good. Just don't pitch shift the guitar, it will sound really bad. Get some thicker strings and down tune it, that's the way to go if you want to tune low.
Alumunium cone usually sound a little more harsh then paper cones, also they tend to last longer. Anyway, if your're trying the cabs just get the one you like better, just try to translate what you hear with a different head to the head you'll be using.
Try the mic with the built-in pres of the mixer and compare the results with your own pre. If you hear a significant improvement do it, otherwise, save your pre for studio use. Just a question, where will you be using the condenser mic?
While there's only one vocal track I usually pan it on the middle. When the harmony vocals appear I would poussibly pan one track 50%-75% to the right and the other 50%-75% to the left. But, as "This Kidd" said, if you could give some references it would help us.
For me, any ernie ball is better than any gibson from the last years, so I'd say that it is a great trade, even better considering it is a family reserve model. Nevertheless, if you can, try the EB JP to see if it fits you're playing and if you like its tone.
I'm sure that the ENGL would suit your needs, although you can always check the Line6 POD HD Pro, from what I've heard it sounds very good. You'd just have to get a FR power amp and speakers or some Powered PA speakers/monitors for it.
The mix isn't that bad, I've heard a lot of worse mixes. The song is cool, the rhythm guitars during the solo could have some work done, they sound boring and are very predictable. If you want, you can send me the audio stems and mix it.
The build quality may be a little bit better on the prestige model, but I think that what you should look is that if you want a fixed bridge or a tremolo, because that's the main difference between those models.
This has potential, you could add some reverb to the vocals, they sound really dry to my ears. You also could add bass and drums to make it sound fuller and maybe some strings in the verses. Has someone has already said, I also think that the guitar is a little too loud in the mix.
Nevertheless this is a great tune, keep'em coming!
You can get a mix table where you would connect the POD and the microphones, the you would connect the "outs" of the table to the interface. Just remeber that if you go this way all the instruments will be on a single track.