May I contribute my 2 cents to your happiness?
1. What Amp. do you use (more as what is it’s DF)?
2. Bi wire is a way to connect two relatively thinner wires instead of one thicker. On Spice (SW) analysis, the results of connecting one cable of #7 awg ( 0.4982 ohms / 1000 ft.) is equal to connecting two #10 awg, of the same length ( 0.9989 ohms / 1000 ft.).
The problem is, that it’s hard to find ready made #7 awg cables, but more common to find #10, #12 or #14 awg cables.
3). 15’ is a bit long for a speaker cable. As longer it gets, so is the resistance increase. So #10 awg with that length might be no so good of a cable.
Please pay attention, that I do not say a thing about prices.
A simple DIY project and cables + accessories, cost less than $100 can work for you. That’s way less than $220 or $5k!
I never thought that cable prices should be high.