I have had the pleasure of working 50 yards from the Jolida factory and United Home Audio store for the past couple years. I just auditioned both the 1501 and 502. Jerred Dunkerson let me bring my source, DAC and auditioning material to the store for a listen. For a short answer, I have decided to buy the 502.
Both the 1501 and 502 are wonderful amps, but the amount of pure A/B power on tap with the 502 is clearly evident. Control, low end grunt, musicality, and resolution are noticably improved with the 502. Given the difference in retail price (about 300), the 502 is clearly a better deal. The only thing the amps share from a tube perspective are the 12AX7s in the buffer stage.
The gear I auditioned with:
Laptop (24/96 FLAC)
Beresford TC-7520 DAC
JD3000 (I think) pre
All cables and ICs were Celtic Silver
If you want to know more send me a PM.