Good answers in both posts above.
I agree that it is a good idea to determine where the "weakest link" in your analog chain is, and then take a step in that direction.
If, in fact, your phono stage is limiting your sound quality to less than the PT/RB300/Clearaudio can provide, then it certainly could stand improvement.
The PT Tarantella is a very good(even if relatively unknown) turntable, the RB300 is one of the best arms for an affordable price, the Black Cube is an "old standby" as one of the best SS phono stages in the lower price ranges.
I agree that there will have to be some serious money spent to better any of these items significantly.
For my opinions, I think a Graham 2.0 or 2.2, or top SME arm is going beyond what the Tarantella can do. You will have more tonearm than you do turntable. If you plan to spend over $2k, then buy a better TT, and put your RB300 on it. You'll have a better result that way, if you choose wisely.
The arm upgrade options are numerous for the Rega tonearms, and that could satisfy your needs without changing arms, and keep costs lower. I think that the full OL or Expressimo upgrades for your arm would provide enough tonearm for most affordable(under $1k)cartridges.
I personally think that the Clearaudio cartridge is the weakest link in your analog chain. That's where I'd look for my first upgrade. Depending on your listening tastes, there are numerous cartridges that could better the Clearaudio in an affordable price range.
So, I think an upgrade package for your RB300 and a new cartridge would be the way to go, unless you plan to spend alot of money, in which case, I'd start with a new TT. The Tarantella is very nice, but only on par with some other $1500-$2k tables, so if you plan to spend over $2k, the TT would be the place to start.
Of course, this is just IMHO, and YMMV.