common problem which may happen:
1) CD tray - not move smoothly, sometimes cd may stuck inside --> try manually test many the eject button (don't only test the remote)
2) If u plan to mode this cdp, be extra careful to plan your mod. it pcb board/solder pad quite fragile. U may not know which contact is loose after you remove the pcb for mod. So better you 1 time modification. If you do many mod from time to time, you may face face a lot unexpected problem. Good luck.
3) If your budget is ok try on shanling CD 300 of other tube model like T80 (cheaper?) my personal view is that shanling's pcb look more solid and better than bada if you like modification very much.
actually bada HD21 & HD22's circuit almost same different is the DAC part, CD pickup.
Bada's HD series sound ok lah at that price, if u don't seldom do mod then less problem too. can check from Bada website and look for their agent in HK.
I'm not sure which part of HK can view bada & shanling as i'm in malaysia, hopefully others here can help u on this.
