But based on my limited experience and also based on what people I know are doing with 300B amplification I had come to the conclusion (for myself) that ideally it should be dedicated to middle and high frequencies.
This is a myth propagated by those who match 300bs incorrectly. While 300b might be more mid-centric than say 211 or GM70, if are running it from 5 to 10 watts you won’t have issues on bass with the appropriate speaker.
The Art Audio 300b amp is also a good budget (relatively speaking) amp, haven’t tried it on Devore but will be surprised if it does not do well.
Now regarding GM70 I will not develop in this thread, may be in your SET thread one of this day. What I will just say is that it is also limited with regards to the bass response but the rest is all magic. I have yet to hear so much weight to the instruments with another amp, but again I am just back to tube amplification...
The GM70 I have been listening to is great on bass
However, it is not the tube itself that is less bass or more bass, so I wouldn’t generalize on how good specific tubes are for bass.
I wouldn’t get too caught up in Kondo or AN. With Kondo at least, you need to spend a lot and build the chain around Kondo to get the magic. Many think buying any Kondo equipment and plugging it in will get them Kondo magic, that is not the case, they will be better off with other electronics. Only if you have the budget and are going all in – Kondo pre (M1000, G1000, maybe M77 (haven’t heard it myself) coupled with a Kondo amp like Gakuon/Kagura etc, you will get there.