Luminous Audio Axiom mkII $199 (passive) - most impressive and significant upgrade in sound quality I've yet done to my main rig. The best explanation I can come up with is the old audiophile cliché: it was like lifting a thick wet wool blanket off my speakers and my system already sounded spectacular and extremely resolving.
technics SL-1500mkII > Denon DL103 (re-tipped by Sound Smith and housed in modified wood body) > Jolida JD9 > Loung Audio Copla MC SUT/head amp > Axiom mkII > NuPrime STA 9 > Tekton Uruz speakers
*the Axiom has replaced three preamps I still have: Accuphase C200, Jolida 3000B and a heavily modified Dynaco pas tube, all of which are fantastic preamps. I have the 'base' model Axiom which means it's just one input and one output and volume control. More ins and outs add to the price and they offer a 'Walker mod upgrade' which is supposed to be amazing but they run from $199-$999 depending on options. Spending $200 to try one is well worth it as I will be getting one with more ins and outs very soon.