Tonearms

No matter what you will do, you will hear alwyas a difference, but I think, the best - or most satisfiying -way is to go for a superior phono stage.
When you can dial in the right impedance for your cartridge you will things, you never heard before ( no matter what kind of Arm etc. you use ).

If your Rega is stock, I would place it roughly at the same level as your Black Cube: a good match. You can certainly get better arms and better phono stages--the trick being to maintain the same value for money, because the two pieces you do have are overachievers.

Have you considered tweaking your Rega? The Expressimo Heavy Weight and arm stub, and a Cardas wiring harness, can squeeze a lot more performance out of an RB300 at relatively low cost. You could then turn your attention to a cartridge or phono stage upgrade.

Origin Live and Incognito offere similar upgrades for the Rega arms but the Expressimo / Cardas one is what I have.

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.