Hey there. I finished my G9 recently and I noticed a the roll off of low frequencies as mentioned in this post. While I noticed it straight away the pre does sound very good indeed and is super quiet. Thanks to Jacob for the open release.
My config is standard(apart from PSU being relocated off the main board) with Lundahl input and output transformers.
After reading through this thread it is my understanding the 6198 tube has impedance trouble in conjunction with the Lundahl 5402 transformer.
I have a spare 12BH7A in my spares box so I gave it a try. I put it in place of a 6189W in channel 2 output position.
First observation a after 20 min warm up was indeed the frequency roll off was less pronounced. Comparing with channel 1 (still 6189W's throughout) yes channel 2 had more low end, however channel 1 with the 6189W's did sound better overall. The sonic character was much rounder, almost 3D in a way. Channel 2 seemed a bit stale and dull with the 12BH7A in place.
I must admit the 6189w's are NOS JAN tubes and are of high quality. The 12BH7A i used is a cheap Russia recent tube. I have ordered a pair of NOS 1950's RCA 12BH7A just cause. If I don't use them in the G9 they'll go into my LA2A's
I did a quick sweep for reference. The sweep is of the same channel, same settings, same warm up time etc. I'll write another post back here once the vintage tubes show up.