Great looking TD-166 MkII! I have the same tonearm as yours on a TD-147. Yes, everything I’ve read indicates that the effective mass of the tonearm is 7.5 grams. In today’s world, that does put it in the low mass category. As several have suggested, the Ortofon OM cartridges work well with that tonearm. Both the OM-20 and OM-30 sound great on my 147.
If you decide that you want to try a low output moving coil cart someday, I’ve found that the Denon DL-301 MkII is a good match for that tonearm. But I have no idea how it would work with your Pioneer receiver. Worse case is you would need to buy a step up transformer to put between the turntable and receiver. I use a Denon AU-320 SUT with my Denon DL-301 MkII. To my ear, the Ortofon OM-30 gets one much of the way to the Denon DL-301 MkII. That said, the Denon gets much more play here than the Orto.
You might want to give some thought as to how you are going to align your new cartridge. I ended up buying a plastic gauge over on eBay made for that arm. It’s easy to use and seems to get the job done. Also, if you haven’t already, at some point, I’d check to make sure the spindle well has some oil in it.
Best of luck with your 166. I’ve come to love my 147.