XMPlay's virtual channel code hasn't changed in many years, so I wouldn't expect this issue to be a new thing. Are you sure you didn't change that setting to 999999 around the same time as upgrading to XMPlay 3.8.4? If so, what was the previous version that you were using, and is there no problem if you switch back to that and set virtual channels to 999999 again?
Anyway, I will tweak the virtual channel stuff for the next release to prevent high virtual channel limits affecting CPU usage, but high limits will still be using more memory, so I wouldn't recommend it. When there are no virtual channels free, it will be the quietest one that is killed/replaced, so you're very unlikely to hear any difference over even 100 voices.