Ugh, the Hafler circuit may have been "revolutionary" in the early 70s when it was discovered, but so was the Ford Capri.
You don't say if you're using the internal tv speakers or not. I suspect the easiest and best increase in sound quality would come from using a pair of external speakers for the L+R channels, connecting them to the audio outputs of the TV - you'd probably notice more of the "surround" that the TV attempts to create. On that note, bearing in mind there's been 40 years of exponential electronic development since the Hafler was developed, there's a good chance that you'll get more advanced surround field processing out of the tv than the cross wiring your speakers in a hafler circuit.
If you are using external speakers and have a spare second set, by all means, try wiring up a Hafler, but i suspect you'll not be too impressed if the surround you're getting out of the TV isn't impressing you - but also try having the rears wired in parallel to the fronts, in serial with the fronts, both in and out of phase.
Your results and preferences may vary... but a proper 5.1 will knock a soundbar and any configuration of speakers and homebrew wiring into a cocked hat -The bay of fleas has second hand a/v amps for less than the price of a night out too, and even a discrete 4.0 system would be leaps and bounds above anything else.
(addendum)
the Hafler circuit only plays the "difference" between the front channels thru the rears. if you've got a 5.1 source, say a dvd, then the tv will convert this to 2.0 stereo for it's own internal speakers (most probably using left and right channels, and adding the centre channel to both of the front pair for dialog, and mixing the rears at a lower volume, possibly with a little bit of a time delay if it's trying to be clever)
Basically, all a Hafler Circuit doing is extending the stereo field and putting the speakers behind you...
Front Left plays left output
Front Right plays right output
Rear Left subtracts right output from left and plays that.
Rear Right subtracts left output from right and plays that.
(BTW, not having a go at the guy who suggested it, i'm just pointing out that it's not going to be groundbreaking, which Stung also said)