Anyone compare Zesto with Backert preamps?

I think it depends largely on what type of sonics you are seeking.  I've read pro reviewers indicate that the Backert (particularly the Extreme) sounds close to solid state - very quick and handling transient well.  This may be what you're seeking, while the Zesto may appeal to those seeking more of a classic tube sound.

My Toolshed Amp exceeded my expectations.  It is a labor of love ,  execution and attention to detail is beyond reproach   

  The preamp is a collaboration between Matt and Gordon Rankin of Wavelength Audio.   I have no doubt it will be just aa awesome as the 300b    

The new Zesto Leto has an even lower noise floor.  Like I said, it is a seriously good preamp and you should absolutely try to audition one.  My Leto was the most quiet component I have ever owned plus it had explosive dynamics.    

I'm blessed to know Bob Backert personally.  He is one of the kindest, most.  centered persons I have met.  He has been transforming my Conrad Johnson PV-12L for the last few decades.  I wouldn't give up my rig for any other line stage!  Yes, the classic warm glow of the cj is gone but the holographic wall to wall soundstage, often attributed to a tube stage is still there.  More transparent, open and revealing than stock.  Bob's big emphasis is on the stability and robustness of the power supply.  He used to, and maybe still does refer to it as the "Greenforce Power Supply" for which he has the patent. 

I know nothing of the Zesto product so can make no informed comments about it.  

but the holographic wall to wall soundstage, often attributed to a tube stage is still there

Bingo! Listening to my system and this is exactly what the Backert brings to it. I love the changes the Backert Rhumba brought to my listening experience. I understand that the Rythm does this even better. And suits OP’s budget.