I'm not sure how to break it down further--You've got a CDP. While it might not quite produce the output you want (you seem to only have your friend's word on it that it will if you trade up for a $700 one), a sub will add bottom end bass. This will matter for a lot of rock and pop, organ music, and probably jazz recorded in the last 30 years or so. The CDP will likely be a minor change; a properly calibrated, quality sub will be a major change.
I did not notice a change in CD quality when I went from my 10 year old JVC (analog outputs) to a modern Yamaha (digital output+SACD, DVD-A, etc, etc.) I have not heard a tube CDP, in all honesty. However, I must take issue with people who say that I can't have an opinion on anything unless I have experienced it first hand.
There is of course the question of whether you want more bass.
