G'day Maties, just wanted to share a few experiences. Had the fortunate chance to try out three different power tubes on similar amps that had no issues with the tube changes vs the transformers.
The amplifiers were not my monoblocks, rather the Classic 60se and 62se.
Both of these are designed based on the KT120 output tube, and the overall sound I preferred was with the KT120.
This I can confirm simply because the Classic 60se was the very same amp I used prior to the LP125m's!
The other tubes were as follows:
Sovtek 6550: very smooth all round sound, airy highs and lush mid-range with tuneful bass lines, easy to listen to and very enjoyable.
Tungsol KT120: the preferred tube, more grunt and power compared to the 6550's, more mid bass slam, full control in the bass and equally extended airy highs, with a very strong presence in the mid-range. Outstanding performance!
Tungsol KT150: this was more fuller, more forward sounding and definitely more heft in the bass but it was not a "balanced " sound. In fact something wasn't right, it was as if the highs, mids and bass operated at three different levels rather than be totally cohesive compared to the others. Not too good a choice but some may like this type of added fullness...
This is exactly the same phenomenon I got when I had the mv60se and replaced the original tubes with KT120's. Although the sound was fuller, it was not well balanced but I continued to use that amp with KT120's. The moment I sold it and got the Classic 60se, which is designed for KT120's, all was mighty fine! Everything fell into place very nicely, perfect match and perfect balance.
Similarly, when I first heard the ART150, that particular type of sound purely matches the kind of tube they use, which is the KT150, nothing else!
Therefore, I can clearly state that these tube types are definitely not for rolling, unless certain types of upgrades have been made on the amps itself, as is the case with my present monoblocks. The LP125's have been upgraded and although originally came fitted with 6550's, they sound superb with either tube. At present I am very pleased with the results of performance from the KT120's. Although these amps can very easily take the KT150's, I have a strong feeling of how that's going to sound...
In summary, unless I was planning on further upgrades and aiming for the ART150 or 300's, there's no point in tube rolling between KT120 & 150, they're nearly similar, with only a marginal difference but on a totally different amp, such as the ART150/300's, they're in a class of its own.
So with that said and done, for those who are trying to tube roll between the two, forget it unless you make the required upgrades. The KT120's are not too far off, and infact all three were marvelous in their own way.
Cheers and have a good one, RJ