EDIT: Apologies for the brain dump... Here's a TL;DR:
1) upright lessons, play that NXT like an upright not an electric
2) switch to the NS Traditionals strings
3) be careful with bass frequencies and you don't need another cab
4) The Zoom A3 seems more for acoustic guitar
5) Try an impulse response or a delay (audio samples linked below)
Bonus tip: Upgrade the stand!
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1) @Mark Gollihur's knowledge base makes a couple good points, the first being that it's important to play the NXT like an upright. Something to think about would be investing in upright bass lessons... Upright is a very different beast than electric bass. I played electric bass for 20 years before getting my NXTa, and still took upright bass lessons. Worth the investment!
2) The other really good point Mark made in his link is the NS Traditionals strings... I bought the
Golluhur edition NXTa after playing a couple normal NXTs and the strings made a significant, very pleasant difference.
3) @DoubleMIDI makes a good point about taking it easy with the bass frequencies. Adding another cab (and particularly a 15") would most likely overwhelm the low end. That Rumble 800 will be
plenty. When I'm not going direct with my NXTa, my amp setup is a single 1x12 cab, and even that feels overwhelming sometimes.
4) The Zoom A3 you mentioned seems like it's targeted at acoustic guitars. It couldn't hurt to try it of course, you may end up liking it! Just not sure how good it would be for an EUB.
5) A different option to consider is looking into Impulse Responses (IRs). Check pairing
3Sigma's Upright Bass impulse responses with something like a
TC Electronic Impulse or
Mooer Radar. I don't care for IRs with electric bass, but have had very pleasing results with the upright IRs on my NXT. Positive comments from both fellow band members and sound engineers.
If IRs aren't your cup of tea, you could also try adding a smidge of delay, and I'm talking a
tiny amount. I had good results with a Simple Delay in my HX Stomp with 15ms delay, 15% feedback and a 35% mix.
I posted a dropbox link in this post earlier in this thread, if you care to check out some audio comparisons of my explorations.
Bonus Tip:
Look into upgrading the tripod stand that comes with the NXT to the metal CR stand. It's spendy, but it's MUCH more robust than the NXT stand. My NXT stand lasted only a few gigs before one of the bolts stripped, and I had to use an emergency packaging tape fix to keep my NXT at playing height. The CR stand is like high end drum cymbal stands.
5sg.