I recently obtained a vintage Ampeg with 7591 power tubes. I LOVE the way this amp sounds, but I fear the tubes may be running hot. It's my understanding that this is a cathode bias amp, so for me that means taking it to a tech to make changes. The amp currently has the original tubes in it. The tubes are glowing blueish when fully warmed up. Either way, I will likely order some JJ's soon and save the original tubes.