My first thought would be to replace the speakers. What is your budget?
My suggestion would be to upgrade your cartridge. I have been using a Dynavector XX2MKII for several years now, in a Scout Signature, and now a DIY Garrard 401/Dynavector DV 501 tonearm. The results have been outstanding. I think the XX2MKII is the sweet spot in their lineup.
Other than that, a speaker or amp upgrade might be in order.
Thanks for the responses. My speakers are clearly the wek link in the chain, and will be upgraded at some point, but my current budget is about $2000, so that's not happening right now. The WA 5 is a suprisingly good 300B SET amp, and I am not in a hurry to upgrade it. This one was custom built and has very high end coponents compared to the average model they sell. But some day...
I see you are invested in the front end of your system but "the rubber hits the proverbial road" with the speakers. If you can save the money, and add to it, there are some really good bargains to be had, for speakers. It appears many heavily discount speakers maybe because of difficulty with packing and shipping. I have seen Wilson Sophias as just one example go for under $5K, so keep the speaker upgrade option in mind. The front end is really very nice, I happen to like the Dyna 20X. I am sure there are plenty of different ?better? sounding cartridges out there. The question is, without better speakers, will that investment's full improvement come through?
I suppose if you are really iching for a cartridge upgrade maybe you buy something that will allow you to still have some to start a speaker fund.
If you have an adventurous spirit, the exploration mood and some courage for a little DIY you may feed your SE300B with the real thing:
A well restored Lenco L75 on a plinth that accepts any tonearm you wish for your cartridge of choise.
For my taste a carefully used Colibri is once and for all.