Oppo CDP is outstanding

Ebay has linear power modules and excellent replacements for the IEC, jumper to bypass the 100/220 switch. The person I bought from has excellent step by step photos on the product pages which show that it's a simple DIY, no soldering  < $500 took my 105 to incredible audio level even for redbook, and since it has a VVC I no longer use a preamp

Try a Vibrapod #3 under each foot.  I found it made a big difference in my 205. 

Hard to tell from your post what you mean by "upgraded" and exactly how or if you are using the micro-rendu.  But if you are using the original Oppo 103 DAC section you could easily improve your SQ by several steps with an external DAC.  In fact, using the software available with the micro-rendu you could probably bypass the Oppo entirely.  A next step might be getting your music out of or running through the computer, where it is subject to degradation, even if you use the computer to control the process.

The Oppo is like a fine Swiss Army knife.  It does a lot of things, but none so well as a dedicated single use device.

Melm,
I had a modest upgrade to my 103 a few years ago by Stereo Dave's shop in Seattle which was an excellent sonic upgrade, but I have no clue what they did to it in terms of upgrades.  I play my 7TB of digital music via my iMac, connected to my computer modem into the Micro-Rendu into my DAC, using Audirvana.  I also have Tidal Hifi.  As my Micro-Rendu awaits a new SD card to hopefully deal with the failure to connect to my computer, I am happy to have the Oppo to spin my CD's in the interim.  I just finished ripping all my CD's onto my HDD so the Oppo is just back up at this point.  At any rate, the Oppo is a stellar performer... for very little money.  Cheers.