Suggestion please: How to improve sound.

Treat the rear wall behind your sytem and maybe a couple feet out on the ceiling and walls. This is the LEDE(live end dead end) practice and works excellently. Room treatment is extremely important and will affect the sound ALOT. Electronics and cables are usually pretty small changes, room treatment is major.

All the above posts are correct and will help. Don’t forget after the above are tweaked that dedicated AC power outlets on their own circuits and clean AC power is also going to make an improvement (after the above suggestions are taken).

I like what's mentioned above. I've sold the 3.3's for years, and am familiar with sound.
What you might want to sometime consider is a differnt amp with those, and even "bi-amping"! If you found an amp who's midrange and trebble where pristing sounding to your ears on those speakers, you could easily pick up something like a Parasond HCA1500, 2200, or 3500 to drive the bass woofer(bottome posts), and a sonically superior Pass X150 or something on top!..which would go good with those speakers probably.
Bi-amping these speaekers would give you more dynamics, and a better "top end" option optimally. Heck, you could even try tubes up top if wanted. Otherwise, there are much better amps out there than the 8008(I've sold and owned that amp). And bi-amping would give you the power and flexibility. Also, try upgrading tubes on the LS16, which I've also owned/sold over the years.
Acoustics, set up, source matching, cabling, power conditioning, and tweaking is the other steps. good luck

I might be repeating what has already been mentioned by other members but what you can do to optimize your system is
(1) work on treating your room. There is some excellent info on the WWW. Use your favourite search engine & type "Jon Risch". You'll learn about tube traps, high freq absorption using panels & lots more stuff.
(2) Work on speaker placement. You can get info from Cardas's website & from the Audio Physik website.
(3) If you have not already, spike your speakers. I suspect that the NHT 3.3 are already spiked by design (?).
(4) Work on isolating your CDP & DAC from your rack. Various methods are available for this depending on your budget. Using ceramic or carbon-fibre cones is a start. You can also build a Symposium-like DIY platform. You can also use an inflated bicycle inner tube for isolation. Some other audiophiles have created a hammock-like suspension system & their CDP & DAC is slung from this suspension. Search the Audiogon archives for more info. In the isolation dept. the sky's the limit in terms of ideas to isolate gear from the rack.

If you pay attention to the room itself, the gains from that are plenty to take your system to another level. It is, sadly, the most forgotten element in most people's 2-ch setup!!

My experience is somewhat dated, but I believe it may be as relative today as it was when I was heavily into getting the most out of my system that I could for the least dollar input. Spikes everywhere may be the most cost effective, and the amp-preamp interconnect is extremely important. Finally going to MIT from Mark Levinson silver, and a variety of copper interconnects provided the most musical and spatial result I found (Note: PV5 pre & Quicksilver mono amps). Good luck.