I just had an interesting preamp comparison experience. My son brought over his phono preamp during the Thanksgiving holiday and we swapped the two preamps in my system. His preamp is a similar design and uses similar components but there are also a lot of differences. As you can see from the photo, his is a 1-chassis preamp with the power supply and audio circuitry all in the same compact chassis. Not so obvious are some other differences. Like me he uses Slagle copper SUTs but he potted his and mounted them on the pream

p chassis; he also uses completely different tubes—RCA 280 rectifier and Western Electric 420a and 396a; he has CCS loading for each tube with cap coupling on the output; and he uses a choke-input power supply with vintage PIO filter caps. Similarities include Copper V-Cap coupling caps and several Audio Note Silver tantalum resistors.
With the higher gain 396a, my son’s preamp has more gain. We are estimating his has about 17db more output based on settings of my AVC.
So how does the different yet similar preamp sound? Somewhat to my surprise, the two preamps were very, very close in sound. Similar tonal balance, and very similar detail and dynamics. To my ears the Western Electric version has slightly more solid and weighty bass and slightly brighter top end, and mine has slightly more delicate detail in the midrange. My son said my preamp sounds a bit mellow and his is a bit more lively.
To put things in perspective, we also swapped output cables on my Slagle SUTs, and the differences between the 3 cables we tried were greater than the differences between the preamps themselves.
The preamp swap was as fair an A/B test as I can imagine. We used the same power cord and other cables and each preamp was located on the same shelf of my Box Furniture rack.
All in all, a very interesting listening experience.