So basically you have a very good analog setup and you want to transition it into greatness. And you've already honed in on targeting a new Phono Stage or
Cartridge, which in my experience are easily the 2 most important components in the analog chain (over TT, tonearm, SUT, and cables). It's tricky to prescribe
which to do first, and in the end of course you'll do both!
If it were me I'd probably do cartridge first, which is akin to choosing speakers before amp (though to be fair, phono stage can and does influence the sound
more than amp). I had a friend in a similar situation with that 17D3/Scoutmaster and a phono stage no better than your Modwright built-in (if not lower in the
food chain). He liked the 17D3 but never loved it, and eventually landed an Ortofon Jubilee which he liked waaaay better (same phono stage). So you can
definitely have success down that path. Also I've personally had satisfaction upgrading my cartridge beyond the pay grade of my phono. Though as a result,
phono stage is still the bottleneck in my current system (what I'm using is a tradeoff: "so there" in midrange, treble and imaging, but "not quite there" in bass
and macro-dynamics).
Upgrading to a higher-end preamp with built-in phono is also a reasonable approach to consider. I had a VAC Renaissance III with an incredible sounding MM
stage (there you'd want to go with a 3rd party SUT for MC though since $1K+ for the inexpensive Lundahls is a huge ripoff); I believe that you can in fact have
a fairly good built-in when it comes to phono stages.
Cartridge, which in my experience are easily the 2 most important components in the analog chain (over TT, tonearm, SUT, and cables). It's tricky to prescribe
which to do first, and in the end of course you'll do both!
If it were me I'd probably do cartridge first, which is akin to choosing speakers before amp (though to be fair, phono stage can and does influence the sound
more than amp). I had a friend in a similar situation with that 17D3/Scoutmaster and a phono stage no better than your Modwright built-in (if not lower in the
food chain). He liked the 17D3 but never loved it, and eventually landed an Ortofon Jubilee which he liked waaaay better (same phono stage). So you can
definitely have success down that path. Also I've personally had satisfaction upgrading my cartridge beyond the pay grade of my phono. Though as a result,
phono stage is still the bottleneck in my current system (what I'm using is a tradeoff: "so there" in midrange, treble and imaging, but "not quite there" in bass
and macro-dynamics).
Upgrading to a higher-end preamp with built-in phono is also a reasonable approach to consider. I had a VAC Renaissance III with an incredible sounding MM
stage (there you'd want to go with a 3rd party SUT for MC though since $1K+ for the inexpensive Lundahls is a huge ripoff); I believe that you can in fact have
a fairly good built-in when it comes to phono stages.