I find Monitor Audio speakers go well with audiolab/Tag kit. How much are you looking to spend? and do you want a replacement speaker package or just to replace the front 2 for better music reproduction? A pair of gold signature 20s would probably float you boat, but they are not cheap, what's your budget? the 20's will be around a grand or more and the 60's I think are £1500. Very big speakers too so you need a big room for them.
As for upgrading the DACs, it probably is worth it, but that depends on how much it costs you and how much hassle it is getting ahold of it. I can't imagine the upgrade being that commonly available now.... you might want to consider selling the tag and getting the newer audiolab 8000AP which is heavily based on the tag you currently have - but includes those better DACs and HDMI support.
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My hifi: Rotel RSP1068, Biamped Audiolab 8000Mx2 and 8000P into Monitor Audio GR 20's. 8000M into GR LCR centre, 8000A (power amp modded) into tannoy MR rears, Sony SCD-XE670, SB Xfi from PC, Sony BBP-S300 Blu Ray, Dell 2407 FWP 24" LCD monitor.