OP, by your description of how you came to purchase the gear, I am going to assume you are somewhat new to the pursuit of higher end audio.
While there may be some merit to the cable suggestions, I would suggest you put that aside from a budgetary perspective for a minute. These suggestions are from the tweakers that already have their base gear well determined and are seeking small incremental improvements as diminishing returns. Fuses are controversial but in any case when you have an AVR as a preamp you are far from the diminishing returns these types of things MAY offer.
Gear is not just about what inputs and outputs they offer. Your AVR receiver has pre-outs for convenience. The preamp on your AVR is geared to mate with the amp section and not necessarily to mate well with any external amp. And no AVR in that price range is engineered to optimize audio for music, and there may well be additional digital processing going on affecting the purity of the music signal.
Similarly, your source is intended primarily for movies. It’s not going to give you the musical nirvana that you mention.
Finally, Maggies will sing with the right power. But hold off on replacing the B&K until you get the upstream components right.
Bottom line, you do need a proper preamp, albeit you might find something quite affordable ($300-$500) either used or in something like the Schiit line of products, and you need a proper source, which in this day and age I would recommend be a DAC and then a source like a streamer or computer that can pick up Tidal or Qobuz as well.
When you get the source to preamp combo right, only then are you in a position to judge the B&K to Maggie combo. 1.7is should sound quite liquid, detailed and nice. If you are still not getting that sound then it could be the amp. I do also believe in room treatments, which can be an expensive rabbit hole but there’s usually some low hanging fruit - just start with speaker placement and toe-in angle. Some people reverse their maggies from right to left or front to back as well.
Welcome to the journey my friend. But part of what you need to learn is when to realize someone is suggesting fifth order tweaks that may have merit only when you have addressed several levels of the basics. Don’t worry about $200 fuses or $500 power cables or speaker cables. There’s much lower hanging fruit for your improvement at this point.
