Subwoofer(s) or Digital EQ?

You have already, a good "gear"(so you said, you are happy with it?)why add anything? If you want to improve something, in the future, start from your source or even get better pre-amp...speakers! You should have less but better, not more and worse! That is just my opinion. Good Luck!

Until 6 months ago I also had the NHT 3.3s. They don't need a subwoofer, especially one that requires an extra crossover in front of the 3.3s. Definitely go with the digital EQ. However, I would recommend t.c. electronics Finalizer. In addition to a wealth of EQ options (parametric, multi-band, shelving and graphic), the Finalizer has several dynamic processors. It's best known for it multi-band compressors, but it also has expanders. Most popular music is overly compressed and as a result has very little real dynamic range. The judicious use of an expander is an excellent way to overcome this common flaw in modern recorders. t.c. electronics, like Z-Systems, is a pro audio company. There products are available world-wide at pro audio dealers. Also, if it doesn't work out; there's a ready second hand market for this item. It sounds like you're having fun with your system. Good luck.

I own a z-systems and have the Sigtech digital correction processor. I wouldn't live without the Sigtech for large scale classical and for very smooth low bas. My speakers are Dunlavy SC V. With the Sigtech I got rid of the uneven low bass in my room and have effectively flat bass to 20HZ. The z-systems is a good unit for fiddling with the recordings balance and as a digital contol unit.I'm not familiar with the t. c. electronics units.