Marantz CD-67SE modifications
The first link has an extensive list of modifications while the second link mostly focuses on upgrading the output op-amps. After reviewing all of the possible modifications/upgrades I concluded that upgrading the output op-amps would probably have the biggest impact on the sound of the cd player so I decided to give it a try.
When I first got the Marantz cd player I analyzed the sound using my Panasonic DVD-A110 dvd player (which has surprisingly good cd sound considering its price) as a reference point. The Marantz was a bit on the bright side with a lot of detail in the midrange. The imaging was okay but slightly inferior to the Panasonic. The soundstage was spacious, moreso than the Panasonic. So at this point the Marantz wasn't much better than the Panasonic except maybe in the category of midrange detail, but at the cost of possibly excessive brightness.
So after listening to the CD-67SE for a few days I decided to go ahead and replace the original JRC 2114D op-amps with a new set of Burr Brown OPA 2604 op-amps. The original op-amps sell for around $0.35 each while the new OPA 2604's cost $4.28 from Digikey so if the cost is an indicator of quality there should be quite an improvement. After purchasing the new op-amps I followed the instructions from the second link above including adding four 3.6kOhm resistors to the outputs to alter the biasing of the chips. After less than an hour of work I had the CD-67SE back together and ready for play. Now I wasn't sure what to expect because the page that describes the modification details only says that there is a big improvement but doesn't give any hints as to what those improvements might be. However, I was able to find a second reference that discusses several different op-amp types and their sound qualities including the OPA 2604 which is said to be the best sounding one for the money. Jon Risch offers these descriptions here... Jon Risch - OPA 2604
I had read that op-amps require a burn in period before they really start to perform but I was a little skeptical at first. Upon first hooking up the CD-67SE I was initially surprised by the increase in bass response. No longer was the Marantz bright, in fact it now had more bass output than the Panasonic DVD-A110. I was a little upset however because there was a lack of midrange detail that used to be there and even the Panasonic sounded better in this category now. Also the imaging and soundstage were average and on par with the Panasonic. So now I was a little bit disappointed because I expected more of an improvement in the sound quality and contemplated taking the biasing resistors out to see if the detail would return. However, I decided to give it some time to burn in and let it play constantly on repeat mode for 24 hours.
Well, after 24 hours of burn in time there was quite an improvement in the sound quality and I was no longer upset. The bass isn't as dominant as before and the midrange regained it's detail but now has a smoothness that wasn't there before (it is kind of like the tube-like sound that Jon Risch describes). The brightness and slight harshness that was there before is now gone but all of the detail is still there. The bass is now deep, tight, and stronger than before but not over dominant... it is definitely not a bright sounding unit as it was before the modifications. The high frequencies are very clean and crisp with a great deal of transparency and air. Now that I've described the frequency related characteristics of the modified CD-67SE I'll talk about the spatial presentation. The imaging is very good now, much better than before and better than the Panasonic. There is also more of a "blackness between the notes" than before. The soundstage is very deep now and is also very three-dimensional. The improved, more focused imaging along with the spaciousness of the soundstage gives the CD-67SE a very realistic presentation that is far superior to how it sounded before the modifications.
So if you have a Marantz CD-67SE, are handy with a soldering iron and aren't afraid to perform some surgery I highly recommend upgrading the analog output op-amps as I did, you won't be disappointed.... just make sure you give it at least 24 hours to burn in the new op-amps before making any judgements otherwise you might not be too happy at first.