I haven't tried this but for 125.00 you can go wrong
Maybe not quite the same, but I have an Opp DAV-H and I output the digital feed into my old CAL Sigma tube DAC. I run the Sigma into my ARC LS-3 preamp and I'm getting beautiful sound from the DVDs (I am running a Doge 6 for red book CD). Right this very moment I am enjoying an Anita Ody DVD (Anita Ody Live in 1963 & 1970, Jazz Icon DVD) via the Opp/Sigma and the sound is PURE GOLD!
I don't really like tube buffers. I've tried the Musical Fidelity X-10 some years ago and hated it.
Transparency is very important for a buffer and the last thing you need is an active gear that distorts your music even more.
On the other hand, I like the Burson buffer.
I saw a used one selling here before. ..
Pointless, especially since you have a preamp section with tone controls built in use them if you wish even though its not the "Purist" approach, but you own the preamp with the controls in the first place.
Beyond that a Good DAC "tube or solid state" would be a better investment. Unless you have to have SACD passing the signal in which case not many DAC's will work for you unless your using the HDMI into a home theater receiver.
Oh and I own the Oppo BDP 83 with a hot-rodded custom built DAC, and yes it is a far better investment than trying to modify or use bandaids. The oppo is an excellent transport and will hold value as it stands.
A Good power cord can also make a difference more so than trying to run the analog signal thru another buffer device. If you mess with the sound just use your EQ built into the mcintosh adding more cables and non-sense in the signal in your case is a losing situation.