Question

No detailed info available in manual, quickstart or, as far as I can tell, video series. Any assistance is GREATLY appreciated!

Answer 1

Technical Specifications for each product is available on the product page. 

https://www.presonus.com/products/Studio-26c/tech-specs

Line Outputs (Mains)
Type	¼" TRS Female, DC Coupled*
Maximum Level	+10 dBu (balanced)
Frequency Response	20 Hz - 20 kHz (±0.1 dB, unity gain, 48 kHz)
Dynamic Range	108 dB (A-wtd)
THD + N	0.0015% (1 kHz, -1 dBFS)

Answer 2

I have the studio 1810 and a audiobox USB. They both are 1/4 TRS