While I was buying my Thumb Bo 5 I tried Infinity NT 5 fretted ( with flamed maple top ) and I immediately felt in love with it!
The sound and the feel of it was incredible ( it feels ultra light, fits your body perfectly with zero neck dive )!
There was plenty of lows in it ( they were well balanced, quick in response and never boomy rather DEEP I would say ). You shouldn't be afraid that it'll lack in that compartment as long as your familiar with ovangkol+maple construction sound. In my opinion it's well balanced bass sound wise and really has incredible amount of versatility to it!
You can really have any sound that you like from it including jazz style pickups sounds ( all from streamer's, corvette's, thumb's in it's own unique sounding voice, and by pulling treble control you can get that TJ humbucker splitted ) ), humbucking ones and that modern Hi-Fi bass sound ( lot's of fantastically tempered middle, middle low and middle highs all coming from that Twin J pickup ) with fantastic warwick growl and the last one is the one that I really liked!
Now I'll try to compare the fretless 4 strings version ( wich I tried couple months ago ) with my corvette fretless 4 strings ( bubinga version).
Corvette is fanstastic example how should fretless bass sound. You can get that Jaco sound from it's bridge pick-up. Neck pick with proper technic get's you into upright bass sound area and it all happens with a lot of mwah and warwick growl. Unfortunately that's all. Considurate it's counstruction we can't really look for more from this bass. This is not negative thing 'coz it really does it's job superb.
And now Infinity fretless...
The first thing when you get that bass in your hands it really stuns you with it superb counstruction and details.
First of all Infinity is bass with lots of sounds for people that were always looking for something other than classical sounds and for the ones that weren't afraid of experimenting with it. And for people that have will and power to use it and control it

So when you buy it I would suggest some quality time to explore the whole pallet of Infinity sounds.
Soundwise it's soooooo much more than any thumb or corvette fretless that I tried, Thumb 5 is as always other story and can't really be compared to corvette 4,5 strings or thumb 4.
First... Neck pick-up only...MAN with proper technic it really gives you fantastic upright bass sound with lots of that beautiful lows ( tone chambers are definitely "explaining" you why are they here

) and that famous "upright clicking". It's mellow, it's harsh, it's anything you like it all just depends on you, your technic and your desires...
Second...Twin Jazz pick up....that's the thing that really pushes the envelope with this Warwick model!!! You can have Jaco tone, modern Hi-Fi sound tone of fretless...anything you like...and it's really sensitive to your right hand technic...
Third...great mixture of sounds with those two pick-ups( there's of course option of splitting humbucker)!!!It's really all up to you 'coz this is TOP of the LINE of Warwick basses...All of the sounds were made with eq. at flat!
And now one personal thing considerating the sound of this bass...
I would definitely go with rosewood for fretboard, 'coz it's warmer and mellower, it's a little more middle oriented than ebony...ebony get's you that highs that Matt0123 was talking about, thou highs on Infinity were never harsh or uncontrolled from my experience...and you can always cut them on your eq... It's shame that Warwick doesn't have podouk for their fretboards...
Infinity is definitely boutique sounding and looking bass from Warwick factory! Thou it can bark like angry dog thou it won't be like mad bull-terrier (Thumb) it'll be more like high class boutique champion with excellent genes and temper

And we all know what are these champion dogs capable of. It all depends on us
Cheers!
P.S. If you choose rosewood I would suggest that you go with natural High Polish finish 'coz it will protect your bass and your fingerboard...