Start off by paying attention to the basics. That is, proper installation and positioning of the speakers, a solid AC system and attention to cable routing, clean connections and proper gear installation i.e. a "decent" rack.
Once you've got that done, get rid of the preamp / tuner combo. Those specific Adcom pieces are a sonic mess. I'm not going to make any specific suggestions here, but i would recommend looking for a preamp that is on the neutral to warm side. Given the Rotel CD player, 555 amp and NHT's, which are very tight / somewhat lean sounding to most people, a small amount of added warmth might not be a bad thing here. I am NOT advocating a direct jump into a tubed preamp though as some Adcom's have lower than average input impedances. Most tubed preamps won't mate well in such a situation and i've experienced that first hand in the past with different Adcom amps. I can think of an SS preamp that i think would work pretty well for you in this installation that can be had for under $800 on Agon almost any day of the week.
Secondly, get some "real" speaker cabling. Your NHT's are capable of very good performance and you've got a decent amp. While that amp is not in the same league as some others mentioned here ( Pass, Threshold, Ayre, etc... ), it is none the less a pretty solid performer. Having said that, using speaker cables that introduce non-linear power transfer characteristics into the equation can make or break a system. I'm going to suggest my "standard answer" here i.e. Alpha Core Goertz MI-2 Veracity speaker cables with their silver spades. Don't forget to install the Goertz impedance compensation networks at the speaker binding posts, which are included free of charge with the cables. The difference in warmth, smoothness, liqidity and finesse of the overall presentation may astound you.
Don't overlook the importance of cabling, but at the same time, there is no need to go berserk either. I have NO idea as to what you are using, but interconnects DO make a difference and are relatively system dependent. If you do a search here on Agon and even over at AA, you'll find that there are very consistent recommendations for several "reasonably priced" interconnects. Some of these come with a lengthy trial period, so take advantage of this type of offer when available. Nobody can tell you what interconnects will work best or you will like the best in your system, not even me ( who is a know-it-all) : )
If you take this approach, you'll end up with a very nice sounding system for not nearly as much money as what you initially planned on spending. This savings could be put back into your system ( the replacement of the tuner or use the tuner from the Adcom, power line conditioning, AC lines, room treatment, etc.. ) and / or more music to enjoy your "new & improved" system with.
One more very specific hint though. Your speakers need to be angled up in front for best results. By resting a laser level
on top of your speaker cabinets, the dot should center just above your head and slightly off to each side of your ears at your seated listening position. You can then change the lenses to produce what is a flat line. The line from one speaker should produce the same height and level of "flatness" as the other. By varying how much the lines overlap, you can regulate the soundstage width, imaging and overall tonal balance. The tonal balance is varied by altering the ratio of high frequency directivity that comes into play due to beaming within the treble region.
I hope this helps and gives you something to think about. Throwing money at a system and replacing components won't necessarily get you better performance although it might change the likes / dislikes of what you already have. This is why i stress having a plan of attack and analyzing the system logically, not just forking over cash at random for a new set of problems. Sean