Interesting topic!
What upgrade did you expect to transform your system but didn’t?
Ca. 2004 maybe, replacing a set of entry level KEF speakers (Q15 LR, I think the center was called Q95c or something like that) with 8” Tannoy duals. I gained sensitivity and headroom but those cheap KEFs with press-in thin plastic baffles (either these or the RDM line were Andrew Jones’s last KEF speakers IIRC) were more refined.
Second, ca 2012, going from entry level AVR power (Anthem MRX 300) to a good and massive separate amp (Sherwood Newcastle A965, which had a
glowing review of its design and performance by Dr. David Rich). Not sure it did anything different.
Was it a case of diminishing returns, poor synergy, or just expectation bias?
First and last. There’s no such thing as “synergy,” though the opposite (mismatch) can be real.
On the flip side, what upgrade actually did surprise you with how much it improved things?
My first equalized multisub system, ca. 2005 or 2006, set up using Dr. Geddes’s methods. The EQ and measurement capability was crude by today’s standards—Velodyne SMS-1 with their MIC-5 multiplexer for spatial averaging—and a huge part of the system budget. Subs could only be individually adjusted for level and phase, and PEQ was global. But that setup was the start of a shallowly-up sloping pleateau, and arguably the bass fidelity I’ve been able to get since then, despite mid 5 figures spent over time on elite drivers, custom cabinetry, and high power amps, didn’t really jump up materially until Dirac ART.
Going from “matching” (which is to say mismatched) LCR to properly identical front speakers, fall 2021. The “timbre matched” set was JBL 708i L/R + JBL Synthesis SCL-3 center, which used the same compression driver and horn design. The identical set added a third 708i for center. This works so well I don’t anticipate a change unless something breaks.
Using CBT speakers for side/rear, also fall 2021. No sound quality dropoff compared to the previous setup, and the big problem (localization to the speaker when too close) resolved.