I would run two 20amp circuits using proper 12/2 cable as specified by National Electric Code (NEC). The jump in cost is the copper only. There shouldn't really be a cost jump in the breakers or the outlets, at least not by much.
The BIG cost is running the wiring. If an estimate is off because of time actually needed for wiring, then you may get up-charged on the labor required.
If this is all new construction and you have open access, then you are getting hosed either way by a trade that... well, isn't my favorite.
You have 3 wires that need to land in the breaker box, times two cables. Just run 20amp circuits for both of them, and yes, put them on the same phase. Not sure how much room you have in your breaker box. There is no reason to go 20/15, just go with them both as 20amp circuits so you can have that added range later on if you happen to need it.
I always run 20amp circuits to my AV closet. I probably need to run a couple more.
I like that outlet. It is overkill, for sure. But, for under 30 bucks, it's a nice piece of gear that looks very well built and will appropriately pair with your Krell amplifier.
Be aware, that if you are thinking long term that you may do a lot more 'Krell' level amplification in your system, then run a third circuit.
The expensive part is running those wires! Not hooking them up. Takes me about 5 minutes to wire in a outlet, and I'm NOT an experienced electrician. Takes me about five minutes to land it on the circuit breaker as well. So, once I have the wire in place, stapled down, and in the box, it would take me 30 minutes total to do both ends for three circuits.
That second estimate seems much more in line if it includes running the 12/2 wiring.
Definitely check on the proper circuit breaker for your box and get one that is appropriate to your needs.