Hi. Until I watched a video today on using a mic preamp before an interface, did not quite realize how much better it sounds, including reduction of room noise. This featured the DBX286s. Other suggestions out there at about same price range !!
 
 

 There are many in the 'Under $500' price category and from what I'm told nearly all of those are made by one or two factories overseas.  Check out this company's modified gear http://www.reviveaudio.com.
 
Also search this Forum.  Mic preamps are/were discussed often.

It really depends on the mic and your interface unit.  A low level mic and an interface unit that is noisy when the amplification level is high will benefit from a (good quality) mic preamp.  But if the interface is "quiet" (is low noise) then a mic preamp may not do anything.   I have a Focusrite ISA One preamp and with my MOTU it's not really needed except with my Ribbon mic.  But I even negated having to use the ISA One with the Ribbon as I bought a "Cloudlifter" in line preamp to use with the Ribbon.
 
 

I don't know much about the DBX unit or how much it costs but a nice external pre is good to have.  At the bottom rung it is hard to go wrong w/ the Focusrite ISA One as above, or for a thicker sound the Warm Audio.  Both go around $400 and, generally, give you more gain than built-in pres.  Both use transformers,  which adds a nice touch to the sound.  The ISA uses a Lundahl, which is a tad lighter and more modern.  The Warm is api-ish for a more "vintage" tone.  The  ISA has a lot of bells and whistles and is great for live singer songwriter, with a pre and DI in one unit (if only using the DI it gets the transformer), headphone output, etc. etc.
 
The warm DI is excellent on bass, esp. sent through to an 1176 style comp.

Warm Audio WA12 
https://www.sweetwater.co...QUaEAQYAiABEgIsYfD_BwE

What was impressive in the video of the DBX was the processing features. The expander/gate when engaged, made a huge difference in eliminating background/ room sounds, and virtually making the vocal pristine and near noise free at lower levels.

but the preamps in that dbx are not that good.
 
better to focus on nice color, or pristine capture, than processing.
 
processing = clownphuckery, in low end equipment.
 

 
if you like to have a coloring preamp,
I can recommend the Golden Age Pre 73.
It´s available in different Versions.
There are 3 Version available, if you can afford,
look for the one with the carnhill transformers :
!. https://www.thomann.de/de/golden_age_project_pre_73_mkiii_plus.htm
 
2. the "normal" Version, sounds also really good, not much difference compared to the plus version above
https://www.thomann.de/de/golden_age_project_pre_73_mkiii.htm
 
3. the junior version
https://www.thomann.de/de/golden_age_project_pre_73_jr.htm?ref=search_prv_8

Great info here folks  . Thanks. Lots of nuances I'm learning..

batsbrew
Warm Audio WA12
 
https://www.sweetwater.co...QUaEAQYAiABEgIsYfD_BwE

This.  I have one and use it as a vocal pre and as a DI for bass, and it is wonderful.