Best budget preamp for my entry level system?

Well, the USP-1 is ~10 years old. The PT-100 is new. Not knowing the comparative build quality or sonics hard to say which way to go. Likely the new one could be returned if it doesn't work out. Both seem to offer much at their price point.

Matching style is a consideration. Only the OP knows to what degree.  

The USP-1 is old, but it uses linear power supply and it looks like it even has a discrete audio stage.  The new PT-100 is all op-amps on a single board with switching power supply.  Also, Emotiva is not as cost effective as they used to be due to move a lot of their manufacturing away from China.  Ultimately, they just don't make them like they used to.

Older equipment is not always bad for 2-channel audio.  10 years is not a really long time.  I would start to get concerned in the 15-18 year area.  I have used and tested a lot of older equipment that beats the crap out of current year stuff.

Unfortunately, 2-channel preamps are always going to be more expensive than other technology and they tend to keep their value.  You could look at some older HT processors.  They were marketed at a very high price, but they lose their value extremely fast.  For example, the older Krell HTS 7.1 processor retailed for $8500, but was sold used for $2k about 12 years ago and can be gotten for around 500-800 today.  The KAV-280p pearmp of the same generation continued to sell used for around $2k even though it only retailed for about $4k, and still has a high $1200-1500 value today.  Point being is that you can get some pretty damn good preamp equipment by buying a old HT processor.  The B&K Ref 20 and Ref 50 mentioned above is an example.

If you look at audiogon, there are several around $300 or less:  Outlaw 990, Outlaw 970, Rotel RSP-1068.  The also have pretty good DACs.  The downside is they never have a phono input for turntables, so you would have to get an external phono preamp, but for the money it's an amazing deal.

Don't listen to the blow hards who want to denigrate what you have. It's why I rarely post here! Your Emotiva will be fine. I've heard them in some very nice systems. Don't see a reason to buy an integrated just to use as a pre. Seems like a waste of money to me. I say buy a Schiit Saga and Mani ( I own a Mani, Modi, and Vali) and sit back and enjoy the music!

I love the look of the late 80s, early 90s Yamaha stuff and it’s Uber high quality equipment. Just got a cx-630 630 with remote for $55 off eBay and it does it all.

  Greetings , I too have Zu Omens, and  would recommend Schiit products . I have a Bifrost, Yiggy , and the Loki . I chose the Rogue RP-1 for a pre . But by the time you get a power cord and NOS 12AU’7’s , it’s too expensive . I rotate a Dennis Had Inspire 12wpc, Marantz 2223B , Sansui 9090, and Dynaco MK III’s. The Omens pair well with clean SS or low power tubes . I would try a Saga and build/buy a pair of the DIY First Watt clones . You will be best served by a pre with a really low noise level as the Omens are such a high DB. Again look at Schiit ! Also on the Loki , I can bump the bass up and don’t need a sub . And when running SS or high gain , I can knock off the top to alleviate brightness. If you can afford the class A Schiit amp , go for it . Look up the article on Schiit at the RMAF 2018. My buddy heard the system and said it was great . And this is from a guy running DeVore speakers and a SET amp . I hope this helps . Cheers , Mike