I have no idea about the T512's capabilities, but it is the lowest model, and also not a current one (think they've just finished with it in favour of the T513. Not sure what the difference between the two.
The current models go as follows (lowest to highest)
T513 Approx £130 new
T533 Approx £150 new
T524 Approx £180 new (Same as t533, but slimmer and Divx compatible)
T562 Approx £370 new (Flagship model)
Soon to be released: Master Series (very little info avail atm)
If you're happy with the player, then keep it.
If you think it's ok, but if were looking for something a bit more up-market, there's other options, including from NAD.
Not sure what NAD do in terms of HDMI. Don't think it's on any of the players at the moment it seems.
Whether Component outs are better; I don't know. Think that's probably a subject for another thread, or there's threads available to help out on this.
In terms of switching all to NAD, I can't comment. I don't know the equipment you're using. One thing if you do decide to take the jump is that you may be hard pressed finding a retailer that can provide a decent demo for you without spending the full RRP. It's never going to be as cheap as Internet prices and I never expected it, but a few enquiries led me to believe it would be a fortune more for the items and quite a few places advised I could get a Demo, but only prior to actually purchasing the items!
If you can get a Demo, see the advantage and willing to front the cash, go for it. Don't know what NAD may have over HK or other way round. May not be worth the expenditure unless you go top of range, that I don't know.
The Harmon Kardon HT amp you have seems to be pretty decent from brief spec I looked at.
I'd say unless you're unhappy with it, probably stick with it as you'd have to spend quite a bit to top it, unless you have some cash burning a hole in your pocket.
Maybe if you have cash to spend, upgrade that cabling slightly. Don't know what Clicktronic are like, but seems price-wise a step below profigold etc so that may be what level they're at. Suggest see if you can get your hands on something a little more up-market and see if you can tell the difference. Would suggest try QED & Chord. Reasonably priced cables with some pretty special signal qualities. Was overwhelmed by what the Silver anniversary did to the mid and treble for my speakers.
You have Silver speaker wire already, so test out an audio interconnect or two and see if that makes any difference. Hopefully you may know someone who has some different interconnects
Clicktronic could perhaps be one of those companies priced below their quality though, so a demo is the only way to see.
The DVD 'Heat' is a great one to test out a HT system. Saw someone write a review on a NAD product and they were right on the spot with that film. Low-volume talking, subtle background sounds, followed by that Gunfight scene outside the bank. Sounds amazing and tests out all the parts a good HT system should shine with.
Bit limited in my experience of various makes, but love what I've got.