I recently was in a local shop that sells the Weiss dacs and had a chance to listen to the 501 and 204. Now granted this experience was not in my own system, but in a very expensive setup nonetheless. I wanted to hear how it somewhat compared to my MKI.
A while back I sent my MKI in for the APS transformers and Vcom mods to determine if I would still feel the need to upgrade to the MKII. After the long and I mean long break in period, it has satisfied my upgrade itch. These mods transform the DAC in all the right ways.
I think it would worth the APS upgrades first and then see if you still feel the desire to upgrade. It saves a lot of money and sounds really really good.