Another AN DAC2.1x Sig owner here. It was purchased used from a dealer and I had it upgraded with all the latest internals this year. I love this DAC, music just sounds so right. This is the first dac I've owned that I can play poorly mastered CDs all the way through.
I've found that the digital cable has a major affect on SQ, it took some auditioning to find what I like. AN DACs also respond well to tube rolling. I've tried different output and rectifier tubes, always NOS.
I wouldn't be concerned about AN DACs being 18 bit. The designers tried different chips but since these DACs sample 16/44.1 and 24/96 through an Analogue Devices chip with no digital filters, there was no audible improvement when going higher than 18 bit.
I upgraded my AN preamp within the family but I don't feel the need to upgrade the dac.