What should I upgrade first amp or preamp?

Karapuz,
Here is a different idea: If you live anywhere near Atlanta, you can spend a fraction of your budget and hire Jim Smith to voice your system. He can come over to listen and on the rare chance that he does not think a voicing session will improve your sound, he will not charge you his fee. I now have heard three systems that he has voiced, and the results have each been outstanding.
If you don't live near Atlanta, get his book or DVD and work on this yourself. You will be surprised at the level of improvement a well-voiced system will reach.
I know that does not answer your question about amp or preamp upgrades. It would be helpful to those trying to offer you advice if you describe what you think your system is lacking.

You may also want to check other threads for speakers that excel at low volume. While the C7's are certainly worthy of your system, they might not be the best low volume choice...

Thank you all for the answers. I really appreciate your help and advices. However, I still can't decide what to do. It's so difficult, especially if there are so many choices. Yes, XA-100.5 is much more expensive than 30.5 and I'll need to spend twice as much. At the same time almost everybody are saying that stepping up from 30.5 to 60.5 in my situation doesn't really make sense because differences are not going to be huge and step up should be to 100.5 to hear the real difference in details, soundstage, etc.
Should I change the preamp to XP-20? Will I see a huge difference? I don't know.
With OPPO I like its capability to play DSD from my hard drive and DAC in it is pretty good.
I'm not changing speaker wires and interconnects, as well as my speakers, I think they are also good.
So what should I do!!!??? :) Any other suggestions would be appreciated.
i guess, first I'll get the SPL meter and check the loudness of my system at the sweet spot and continue from there.
Thank you again.

All your stuff is very good, and there is no right answer, just your personal preferences. I have had the XP-20 for a few months and it is awesome. Can't go wrong with that upgrade.

I’m also thinking the XP-20 due to the much larger power supply. I say this from experience with another Nelson Pass preamp, the Threshold FET10. I owned the original, then added the larger upgraded “e” power supply, and then upgraded to the FET10e. The power supply upgrade was the most significant step up in overall performance between the two changes.