Jea48's right. I got the importance of the front and back rows confused. My bad. Also, he is right that you should try his suggestions, first. (They're a heck of a lot cheaper, for one thing!) I don't know why, but I thought you had told us you had done that tube swap, already.
I'm not sure if Herbie's Ultrasonic dampers will give you more bass, or not. But your tubes will, definitely, sound better, less harsh. Maybe more apparent bass. (I use them, myself.) Actually, Herbie does make a specific type of tube damper to add bottom weight. It's called the "Peek" series. But, I haven't tried them, yet.
I'm not sure if Herbie's Ultrasonic dampers will give you more bass, or not. But your tubes will, definitely, sound better, less harsh. Maybe more apparent bass. (I use them, myself.) Actually, Herbie does make a specific type of tube damper to add bottom weight. It's called the "Peek" series. But, I haven't tried them, yet.