Before you get into different tubes/preamps I'd play around with your speaker positioning. Make sure the tweeters are at least 6ft apart and if you have them out in the room, move them back towards the wall a touch. Also try moving your listening chair back (if possible).
The other issue is that "thin" usually refers to a lack of bass frequencies between 60 and 250 hertz, but I wonder if you're using it to mean too much treble? Do you find the sound a bit forward/piercing, in a way that drowns out the midrange because it's so in your face? If your ears are sensitive to and/or damaged in the 2-6khz range you'll need to be very careful with your set up. The easiest way to test this is to drape some tissue paper over the tweeters which will attenuate those frequencies without affecting the 10khz plus.
The other issue is that "thin" usually refers to a lack of bass frequencies between 60 and 250 hertz, but I wonder if you're using it to mean too much treble? Do you find the sound a bit forward/piercing, in a way that drowns out the midrange because it's so in your face? If your ears are sensitive to and/or damaged in the 2-6khz range you'll need to be very careful with your set up. The easiest way to test this is to drape some tissue paper over the tweeters which will attenuate those frequencies without affecting the 10khz plus.

