Wow, glad to see that this thread is going strong.
I have owned Octave MRE 130's with Super Black Box for about 6 years now. I have done extensive tube rolling in these amps. The original factory tubes were Svetlana Winged C KT88's. These are by far one of the best sounding with the MRE 130's. The second best with the MRE 130's are the EH KT88's. Forget about the JJ's as they are one of the cheapest made and aweful sounding tubes I have ever had in the MRE 130.I tried the Gold LionKT 88's and they are good at providing a linear and balanced soundscape. The EH's have more high end energy and are precise in their presentation. The Svetlana Winged C had the best and most pleasant midrange, with balanced bass and highs. I also have tried the Tung Sol KT 120's and they are also a good sounding tube, perhaps a smidgen more bass. I like them because I believe they do give little more kick, but they lose out to the Winged C's mids. I have a Tube Amp Doctor Bias Master to check bias of each tube, the KT 120's require a little more to be ideal, however, the MRE 130's are averaged to be about 39-44 ma per tube. The KT 120's need higher to reach full power potential, you can run the bias a little hotter with the measurement tool to get the added power required of the KT 120, otherwise, you will be safely in the range of the KT 88's bias range. Luckily the MRE 130's are robust enought to allow for such experimenation. I over biased a set of KT 120's when I was trying to burn them in and set the ma to 70, way to much for these.
I would recommend that you do try finding some Svetlana Winged C's and compare them to some EH's. The EAT KT 88 tubes may be good, never heard them, but I have heard the Psvane KT 88's, and did not like them as much as the others. I believe they are cut from the same vein. The TAD KT 88's are also derived from some Asian markets and sound remarkably close to the Psvane KT88's. TAD offers select versions of the KT 88 that are also really good.
Yes, it is true, the plate voltage of the MRE 130's are 530volts, the V40SE : 480V , V70SE, V80 : both 520V.
I have also experimented with the the drive tubes, I used RFT's, Siemens, and Philips-Jan (original Octave), and Psvane 12AU7's. The 12Au7's are really fast and provide plenty of high end energy, perhaps too much in some setups. I really like the RFT's.
I never owned an HP 300. I have owned the HP 500SE with phono; however, I now own an Octave Jubilee preamp. The HP 500SE with the MRE 130's is a match made in heaven, in fact, one of the reasons I was reluctant to upgrade to the Jubilee, because the HP 500 SE sounds so good. I was able to jump on a super deal while stationed in Germany on the Jubilee, and the rest is history.
The Super Black Box is such a tremendous upgrade, I find it would be almost a waste of components without using the Super Black Box. Image and stereo separation will collapse, along with the bass taking a big loss. In addition, the Super Black Box serves as part AC filter, so you do get cleaner filtred power.
I have been to the Octave shop several times for both business and pleasure, and each time, I am amazed at the quality of the Octave products. The Octave get togethers at the Octave factory are a great experience. They happen towards the end of summer.
You can read about my reviews and experiences in regards to tube rolling the Octave and Aesthetix IO Signature phono amp on Audiogon and Audioasylum.
BTW, I drive B&W Nautilus 800 speakers with these amps.
I have more experiences to share with you all later. Just getting settled back in the US after spending 20 years in Germany.