Best advice.
Hi Vegas,
The Denon AVR-3808CI is an excellent receiver, plus an excellent match with your Klipsch speakers. I highly recommend it for your situation. It should be around $900. That's the price they are now going for.
A second subwoofer would be the best choice, before getting an external amp.
It will allow you to have a much better low bass sound balance in your room.
This is extremely beneficial, more than an additional amp.
The Emotiva XPA-2 is a dynamite amp, and at rhe sale price, it is a fantastic deal, so is the XPA-3. But the XPA-2 amp is the better one, with true power to spare. But you might be just as good to go with the XPA-3, because your Klipsch speakers are exceptionally sensitive, plus you get the extra benefit to power your center speaker, plus at the price right now, it's a no-brainer.
The Rotel RMB-1095 is a superb mutichannel power amp, extremely well build with amazing lab test results, plus lots of clean power with great sound quality. A very reliable amp, from a solid reputable manufacturer.
Just cost more than the Emotiva amps, but you might be able to get an excellent deal at Audiogon, so keep your eyes and ears open.
The Parasound 5250 is another great amp by again a great company.
And again, you might be able to get a great deal for it.
* You sure know how to pick the best valued amps around.
And you do make my job harder on which one to recommend you first.
But like I said, go with the sub first. Getting the same model subwoofer (JL Audio Fathom f113) as you already have will be the very best choice, for the exact perfect balance in your room.
And if your room is not HUGE, pick the Emotiva XPA-3 (only $539) to power your three front main speakers, then your new Denon AVR-3808CI receiver for the four remaining surround speakers. That way, you'll save the most money, and you put audio performance and wisdom first.
Remember, you got Klipsch speakers, which are easy to drive with their very high sensitivity.
Your 'leaning' into having a second sub is the right lean, that's a true smart choice.
Trust me, I know, I got two of the same subs in my room. And the bass is much more balanced at more listening positions. It just works wonder at equalize the equilibrium of peaks and nulls in your room.
Hope this confirms what you seem to already know.
Cheers,
Bob