The Markbass R500 is a bass amplifier head that will have you
  rocking out old-school, rocking hard, creating your own
  alternative rock, or shredding up the stage stealing the
  spotlight from your guitarists! The 500W analog R500 amp (at 4
  ohms; 300W at 8 ohms) features a tube preamp that will break up
  just the way you want it to, and a newly designed limiter that
  produces glorious distortion instead of cramping your style. The
  simple 4-band EQ gives you all you need to access a wide range of
  sounds for any genre. As far as controls, the Gain knob is for
  the amount of tube overdrive, whereas the Master knob is for your
  volume.
  FEATURES
  - Analog amp with a tube preamp which provides 500W at 4 ohms;
  300W at 8 ohms
  
 
  - 1/4" input jack works with passive or active basses
  
 
  - Gain knob controls tube overdrive
  
 
  - Master knob controls volume
  
 
  - Line out controls rear XLR line out volume level
  
 
  - 4-band EQ: Low, Low Mid, High Mid, HIgh
  
 
  - Neutrik Speakon Combo out jacks for use with 1/4" or Speakon
  speaker cables
  
 
  - Balanced XLR line out
  
 
  - Pre EQ/Post EQ switch to determine whether the output signal
  is effected by the EQ settings or not
  
 
  - Ground lift switch to eliminate hum
  
 
  - Tuner out sends unbalanced signal sent to a tuner, amp or
  recording equipment not needing a balanced signal
  
 
  - Send and Return Effect ensures main signal continues even if
  something dies in your effects chain
  
 
  - Dual-sided fiberglass PC boards with lined perforations for
  high-quality solder connections
  
 
  - Surface-mount components are silicon-sealed to isolate
  vibrations
  
 
  - Computer-grade pre-assembled ribbon connector cable harnesses
  and ribbons for noise-free operation
  
 
  - Integrated threaded posts for easy assembly/disassembly and
  added durability