The U5 is not tube - it is a quasi-steril (clean) high-voltage Solid State DI with some tone shaping built in if desired.  It is a GREAT DI and particularly shines on its low-end tightness (practically goes to DC!), but not exactly a "Flavor Saver".  For warmth and harmonics, I'd lean more towards something like a Redd DI or similar. 

I picked up an older HHB Classic-80 tube pre that does wonders on bass when driving the tubes hard (crank the gain for warmth/harmonics, reduce the output to the AD to avoid clipping).  I use one channel for DI (U5) and the other channel for the Bass Mic (421) - the combo of both captures a good deal of rich harmonics that help the bass hold up in denser mixes w/o getting lost and w/o being overly bass heavy. 

There are also lots of good colorful/personality mic pre's that also have DI inputs on the front - you get dual functionality out of them instead of buying a dedicated DI...

The SansAmp Bass Driver DI thingy also gets rave reviews for adding just a bit of "Dirt" to make the bass stand out w/o being "pokey"...

Of course, a GREAT bass played well is the obvious starting point...

I'm not familiar with the Octamic-II, but most any Mic Pre I'm familiar with has a "Line Input" in addition to the Mic Input - or at the very least will have a -20dB pad to facilitate hotter line level inputs.  Most dedicated DI boxes spit out a quasi-mic level signal anyways that actually NEEDS a mic pre behind it - but some do send out hotter line levels that don't require further gain.

This is all subjective, and there is no "Buy this mic if you want a warm vocal" or "buy this DI for a killer bass sound".