Being completely honest, and ABSOLUTELY no disrespect.
The difference in audio that you would even get for DD 5.1 would be much better spent on your speakers ATM, rather than a cheap AVR.
What would be the sense in giving TrueHD, to very low cost speakers.
I would work with the AVR you have, to get absolutely GREAT 5.1 Dolby Digital with much better speakers.
You could get a $1k AVR, and it is not going to do anything for the current speakers, or absolute minimal difference at best, which only would be because of better speaker management and sound modes.
But adding sound modes, is quite generic (even on a high end AVR).
And the better speaker management, you can do yourself with a SPL meter and some well spent (free) time, to setup correctly.
As far as the timbre matching of the center that you have and the mains, and I have NO IDEA what the other fellows said (I did not even look for the thread).
But from my best guess, (and this is based on quite a few setups I've had), that the matching would be well off.
You would be much better of with 3 of the same speaker, or at least 3 from the same series.
The subs mentioned are all good, (I only briefed over the suggestions).
But for a good budget sub (as well speakers), I would also consider the BIC H100 from the Acoustech line.
I had one, although not great, it does give quite a good punch for HT.
That one I tested against a JBL sub of the same price range, and for HT it was much deeper.
I am kind of a fan of HSU, but I do know the cost is not for everyone.
I've had 3, and just about to get a 4th, but may try the new ED subs (and test it against my current HSU)
But that sub I am thinking about is 1300.00 on its own (and most certainly is
NOT needed for WOW factor in a well setup 5.1, 6.1, or 7.1 setup.)
Contingent on room size, many budget subs will give EXCELLENT wow factor.
But once again, my opinion (and it is only MY opinion) is to work on your 3 front channels first.
And pick up matching surrounds when funds allow (if budget is a concern).