Can you explain more about the caps? What do they change in the sound? Where did you get them? About how much should one expect to spend? What are the details I need to be aware of when shopping? Thanks!!!
The 1uf output caps in the stock JD9 are low quality Chinese parts. The output capacitor quality is critical as it is in the signal path going to the next stage. Most people replace these caps with good quality film caps. Jolida upgrades them with Clarity Caps which cost about $5 a piece. Lots of people use Obbligato Gold for this mod ($14 a piece). I chose Mundorf Supremes. They are excellent. You can spend a lot more and get "better" but the Mundorf Supreme is good. They cost $20 each and you need 2 total (one per channel). I bypassed the Mundorf, which is completely optional, with a 0.01uf Vishay Roderstein MKP 1837 film caps. They cost $1 each. I got all four caps at Parts Connexion, $42 bucks total. You can spend less or more, depending on how fancy you get.
How do they change the sound? Well, everything improved with them installed and I think the improvement was more noticeable than the Op Amp swap. The highs become more liquid and airy, details more present, sound stage better, everything gets smoother. The Mundorfs are much better than the stock junk that Jolida uses, they are also four times bigger in volume. Scan the thread and you can see how the better films caps are installed to compensate for the size difference. Make sure you know how to install them before you buy.
What are the details you need to be aware of when shopping? Shipping costs - Order as many parts are you are going to need looking across multiple project to leverage the shipping costs. Ebay Seller's - some of these guys sell used parts and some of them sell fake one. Unless the Seller is verified stick to the online retailers.
Good Luck and enjoy! Bob
PS - the biggest bang for the buck improvement are the 12AX7 tubes. I have tried many. I bought my JD9 used for $200 bucks and it came with stock Chinese tubes that were lousy. Of the new production tubes I have tried Sovtek LPS, Russian Mullard copies, Tung Sol, and Tube Store brands. I liked the Sovtek LPS and Tube Store Preferred Series 7025 the best as they seemed to be the smoothest of the bunch. The reissue Mullards were grainy, the Tung Sol too bright and forward for me.
By far NOS 12AX7's are much better. The classic ones like RCA Black Plates, Mullard, and Telefunken sound the best to me. They cost a lot. All of mine were acquired as used NOS and they still are way better than the new stuff. Tubes make the biggest impact on the sound.