The gain knob on your plate amp is likely also an attenuator, even if they label it as a gain adjustment. Amps generate a static voltage gain to the input signal, typically 26dBU, 32dBU, etc. depending on the application. So irrespective of the knob setting and if it’s mounted to a plate or inside a box, the gain being applied to the signal is static.
When you add another box like you describe you are basically adding another amp stage, which just has its own much lower static gain, plus its own attenuator knob. Any noise added in this stage is also being amplified by the next amp and might also be more input voltage than your existing amp can handle, requiring you to then attenuate back, so you should be careful and ensure you really need it.
Display 2013 Samsung 75" UN75F6300 TV Media Xbox One X, Dune Solo, QNAP TS-453A NAS
Pre/Amp Marantz 7702mk2 Processor + DIY 11ch Icepower 50ASX2BTL Monoblock Amp Amp Build Link
7.2.4 Speakers DIYSG 1099s (LCR), Volt 10LX (SL, SR), Volt 8LX (BL, BR), Volt 10LX (TF, TR) Volt Build Link
Subs Dual 19Hz DIY Ported UXL-18s + FP14k Amp + MiniDSP 2x4 Balanced Tactile Crowsons + iNuke 6000DSP