Sealed box speakers work well for me in a small room. I have heavily modified Yamaha NS-1000m. Now semi active, with replacement Scanspeak 12" bass drivers. Incredible bass depth, detail and control in what is a poor room. Not boomy at all, even at high levels. The Yam Beryllium domes are legendary and do the mids and top end justice. The stock Yams need careful amp matching, or they can sound top end forward.
I've had a few SL-1200's, its ok, but not a brilliant deck, lightweight platter and the bearing is only adequate. I've done many, many mods over the years, including Hot Rodded Rega arms and a Transfi Linear tracker. To me this is the weakest link and rather than try and upgrade it, I think a different TT/arm is where you should start. Possibly looking at speakers, better able to work in your room, as a second stage.
If you want realism, on anything other than small group acoustic. You need depth and dynamics. In a small room this is difficult, I've found sealed box is the only way to achieve this and retain any kind of control or quality to the sound.
The speakers and how they interact with your room is a critical factor. A "good" speaker may not be good in your environment. You have some nice bits of kit, I feel the room has to be seen as an issue and dealt with appropriately. Fundamentally the speakers, or rather type of speakers, will have more effect than any room treatments you apply. This is presuming there is some form of floor covering and furniture in there. My choice for your environment, best to worst would be:
Sealed Box
Panel
Ported
Open Baffle (due to space requirements).