AS an overall guide towards an investment strategy to achieving your goals of improving your soundstage, imaging and overall musicality - Text Book - one should invest about 25-30% of the total budget into cabling/power delivery. Power is the most important foundation of your system.
I would say that the MRSP of what you have is upwards of $70K, which means that an investment of about $20k MRSP should have gone to towards that end of the budget.
By using the used market, you can definitely shave that $20K down substantially and probably fall into the budget you have for investing in an upgraded preamp. Ultimately getting a much better return on your investment.
As far as Diminishing Returns - I believe that is purely based upon your personal goals. If your goal is purely financial then this is the wrong hobby for you. If you enjoy music and are looking to enrich your life with the artistry associated with audiophilia the better the return.
As a fellow middle class guy I would say take your time, educate yourself on the products that are out there. Stay true to your goals and remember there is no one way to achieve them. Most of all be patience especially in dealing with a used market. AND ENJOY!!!
Check out:
Thales Cables - best signal/speaker cables for the $$$ period.
Also great Turntables.
http://www.tonarm.ch/en/products/accessoires
http://www.aaudioimports.com/ShowBrand.asp?hBrand=30
https://www.usaudiomart.com/details/649309042-thales-precision-interconnect-cables-rca-to-rca-15-met...
HB Cable Design
http://www.hb-cable-design.com/1_english/ps_2.html
High Fidelity Cables
http://www.highfidelitycables.com/products/power/ct1u/
https://www.audiogon.com/brands/high-fidelity-cables
Stage 3 Concepts - Power Chords are their strong suit.
http://www.stage3concepts.com/PRODUCTS_main_page.htm
Analysis Plus
https://www.analysis-plus.com/
Oyaide MTB 4 or 6 R1 Power Distributor
http://www.oyaide.com/ENGLISH/index.html
Audience
http://audience-av.com/conditioners/ar6/
https://www.usaudiomart.com/details/649484119-audience-ar6t-power-conditioner-wo-power-cord/