That is indeed a very good deal for the DX300. Did you say it came with the stock amp card? Which one is that the Amp11 MK1 or Amp11 MK2? It is getting increasingly harder to get the MK1 card nowadays. It was in production for only a short time before being replaced/upgraded to the MK2.
If you want a completely different sound (or several), I do indeed recommend getting the Amp12 card. Not only that, but it has a full 1W+ more driving power. If you want a fantastic tube out (though it is only SE connection) I recommend Amp13 as well. It has much less driving power than even the stock amp card, but if matched with the proper head gear, is absolutely brilliant IMHO.
I may or may not be missing out on something great, but the Amp14 card sits somewhere in the middle of Amp13 and the stock amp card, and I haven't really felt the need for it to this point.
Congrats on your new shiny!