Can go from under $200 - Radial JDI (passive), J48 (Active)
Under $600 - A Designs REDDI, Demeter VTDB-2B Tube Direct Box (used)
I'm sure there are plenty of recommendations above those prices as well.
Reamp boxes - same idea - your budget will dictate. I've used the Radial Pro RMP and have been happy with it but on guitar.
nedorama Monkey Boy Studios
Summit 2BA-221, TLA-50
mBox Pro 3, Pro Tools 10.3.5, OSX 10.8.4
Radial JDI x 2, ProD2, ProRMP
'65 Tremolux, '65 Bandmaster, '65 Showman; '74 Princeton, '77 Princeton Reverb, Dr. Z. Mini Z Head & 1x12 Cab, pedals, George L's cabling
'65 Farfisa Compact Duo www.facebook.com/thetoysband