I play mostly oldies, so I rarely need that Hi-Fi sound, but when I need it, it's a combination of active preamp, bass and treble boosted a little, and a hifi sounding amp, such as an SWR or Eden, with the EQ set flat on the SWR, or the low mids cut 2-3 dB on the Eden.
Yes, what Jeff said. Eden. D'Addaario Slowounds. Lakland, Sadowsky, Fodera or Pedulla (and many others).
An active bass that differs totally from a Fender (neckthru, quarter-sawn maple neck, ebony fretbord, Bartolini soapbars, 2-band EQ). Stainless steel strings (.035-.095) Low action A good preamp/poweramp combination (solid state). A cab with tweeter Rackmount compressor (optional) BBE Sonic Maximizer (optional, but addictive)
I'm not sure what we're talking about here - e.g. studio or live? If in the studio then it's usually just down to the bass going direct into the desk. Live - I would agree with the others about Eden Heads; but if we're talking "Hi FI" then you can't ignore Euphonic Audio cabs - the most Hi Fi bass cabs I've ever heard, with their additional drivers and tweeters in Appolito configuration!!
Alphonso Johnson describes his hi-fi solution on his message board. http://www.embamba.com/wwwboard/messages/1879.html
My new recipe; add 1 bossa 6, a good cord(all I don't have) a demeter pre, Crown micro1200, 3 Wayne Jones cabinets and just a touch of a fretless F-Bass.