Of course most of what one hears from a speaker is the off axis response (that’s what fills the room) so that is important but unfortunately seldom shared with customers in a useful way (eg, using the CEA2034 format).* So just looking at output won't get you great sound. But you do need output so we can start there.
But perhaps more to the point, we don't know the seating distance for this theater, so it is all guesswork (even if we assume that reference level output is wanted) since seating distance is a key variable in figuring out what speaker and amplification are required.
I'll take a guess that since the room is roughly 18 by 17 feet, the main seating is probably about 10-12 feet from the front speakers.
Let's conservatively assume a c.100 watt per channel AVR / amplifier setup.
A question that frequently comes up when designing a home theater is 'how powerful an amplifier do I need?'. The answer to the question depends on a few things, in particular how sensitive your speakers are and how far you sit away from them. When we design a home theater we want it to meet...
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Looks like you'd need a 93db efficient speaker to reach reference levels for the front row.
And perhaps 96db efficient for the rear of the room to get blasted that much.
That does suggest you need really efficient speakers!
Something like Triad Gold LCR have 92db efficiency but can also handle more than 400 watts of power. Those would work for you but unless you find them second hand might eat up too much of your budget.
The PSA speakers you mention will have more headroom and take less power to drive and cost less, so that would be a reasonable choice though again at 1k per speaker your budget may have trouble? The THX you mention have the output. But none of these have good data about their response in a way that doesn't obfuscate the details you need to be sure you are not just getting high output and okay on axis response. (Example, look at the published measurements for the Klipsch. Can you tell whether the off axis response mimics the on axis response or just that it varies within an audibly wide window? I can't.)
You might also want to add Procella to your list of speakers to consider, as well as JTR (which, on the used market, might also fit your budget, and have huge output).
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You've got one great sub already. It is MUCH easier to integrate two subs that are identical, so I would just get another one of those.
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*Summary of some key studies about how people tend to prefer speakers with good off axis response: http://seanolive.blogspot.com/2008/12/part-3-relationship-between-loudspeaker.html