Well, you spent serious $$$ on the speakers — not so much on the integrated. Not saying the Yammy’s not a nice amp, but driving a $15k pair of speakers with a $2500 integrated is, IMHO, a bit of a mismatch. Not only are your speakers likely underpowered, as you’ve seemingly already determined, but you’re also using the preamp section of the integrated that by necessity had to be designed to meet that relatively low price point, which to my mind isn’t in the league of doing full justice to your excellent speakers either. Point is, despite adding another amp you’ll still be performance limited to some extent by the preamp section of the Yamaha. Or, put another way, it’s a continuing bottleneck to better overall system performance. (If there’s one thing I’ve learned about building an audio system, it’s to NEVER underestimate the importance of the preamp.) Plus, as DeKay alluded to, you’re potentially opening a can of worms by bi-amping with two different amps.
So, if I was in your situation I’d sell the Yamaha and use that plus the funds you were going to drop on another amp and buy a better used integrated that will both up your power and likely give you a better preamp section as well. One potential option would be a Hegel H360, which is a $6000 integrated, and there’s a nice one available now on US AudioMart for $3500 (or you could get an H300 for under $2000). I think your PL300s would significantly appreciate and reward you for upgrading to an amp of that caliber.
Sorry if this seems a little forward or harsh as that’s not my intent at all. I just think your speakers could give you a lot more of what they’re capable of if you feed them with even better upstream electronics. Best of luck in whatever you decide.