Tube Amps With Good Bass Qualities

Agreed on the Line Magnetic. The LM-805ia - which has replaced the 508ia but with similar specs - lists for the OP's 5k price point and has a negative feedback control with 4 positions.

Manley Mahi Mahi mono blocks.  These are scary good providing a solid foundation for bass.  They are very quick with EL 84 tubes and get the midrange right.  These blast away so many stereotypes falsely attributed to tube amplification.

Another vote for Atman-Sphere.  I have the M-60s with the upgraded power supply driving speakers that are basically 10 Ohms (minimum is about 8, I think), and the bass is deep and articulate.   I couldn't be happier.  But as has been well articulated above, you have to have a happy match between speaker and amp.   If you don't get this right you are going to have issues.  BTW, these amps are gems from top to bottom.  Get the right tube complement and these amps sing like angels.

@brownsfan

Another vote for Atman-Sphere

I completely agree, and I will also add that most speakers with  drivers presenting a 8/10 ohms load (or more) "nominal" will provide good bass qualities for most tube amps, that's the "happy match" you mentioned. I have MA-1's and love the sound. BTW which "right tube complement" you have (power and driver tubes)

@luisma31, I've stuck with the SED winged C power tubes.  I haven't seen anything likely to be better for any kind of reasonable cost.  I've done a good bit of tube rolling on the 6SN7s, using a few NOS RCA, Sylvania, and GE tubes, as well as a few different current production brands.  Right now, I have a Shuguang WE+ at the position requiring the GTA/GTB tube.  I've tried other tubes in that position, including various NOS RCAs, Sylvanias, and a GE GTA, and I like the Shuguang WE + best.  It is a very large tube, and it will only fit in the isolated front position because of its size.   In the rear positions not requiring a GTA/GTB tube, the best I've found in the rear middle is a NOS Sylvania Bad Boy.  Every other tube I have tried in that position results in decreased dynamics and top end sparkle.  For the rear row right and left tubes, I really liked the early 50's NOS RCA grey glass.  I recently picked up one of the new Sophia Blue Glass tubes and rolled that into the rear right position.  I really like that tube, and I am thinking about getting another pair for the left rear position.  Those are very nice tubes!  Sweet and smooth.  I don't know how the various tube locations in the M-60s (MK 3.3) correspond to the MA-1 amps.   I'm curious about your experience with the 6SN7s.