Prehaps the firmware has something to do with it?
Other than firmware what else has changed? Cables? Configuration?
If you go back to the original firmware and the exact same bag configuration and change one thing at a time perhaps you can find the culprit.
You should stick with the 200 series receivers (QRX200, QRX235)
The 235 has the option for a built in Zaxnet transmitter - but if you went with the wideband 200 you can use a standalone Zaxnet transmitter like the ifb200 or cameralink with Zaxnet TRX900CL.
The zaxcom will give you wide band, never clip, internal recording with time code, wireless remote control (input gain, frequency, output power level, sleep mode, recorder transport).
If you get a QRX200 and Camera Link (TRX900CL).
The advantages are
wideband receiver which gives you the ability to use all 100 MHz vs a block specific 35 MHz section of the spectrum.
A ZaxNet and UHF transmitter all in 1. So if you eventually want to add a RX200 and send 2 channels of audio to a camera with TC you can. In the mean time you can just shut off the UHF transmitter on the CL and have a ZaxNet TX to control transmitters and send audio and scratch track to the ERX.
The CL adds a two channel backup recorder with TC in you bag. I roll my all the time - you can never have too much redundacy.
The disadvantages
You need to buy a QRX200 and TRX900CL vs just a QRX235 and add the QIFB
You need to fit two boxes in you bag vs one.
As for the transmitter that will depend on your work flow.
The LA's are great because AA batteries are easy to find and you will get around 7 hours of run time with a pair of lithium's. The ZMT will give you a bit over 5 hours with a NP50.
The ZMT will give you slightly better audio quality.
The ZMT is about half the size of the LA.
The ZMT runs virtually cold
The ZMT will max out at 75mW vs 100 on the LA (regardless i almost always run mine at 50mW)
Other than that they have the same features and functionality.
As for the BNC mod that is something that the Audio Department in Burbank CA created - but I do believe there are others that have copied.
https://www.audiodept.com/
It the transmitter actually a stereo unit?
When you boot up hold the down key and the screen that displays the firmware version and serial number will display the product number.
Essentially never clip is comprised of a dual A to D converter where one converter is running lower than the other. Then just before the signal clips a special look ahead algorithm will force the signal to be routed to the lower converter. Then the never clip software mixes both signals together so there is a seamless transition.
While I'm not using a 633 I'm using both Zaxcom and Lectrosonics side by side in the bag.
I find the Zaxcom range is equal to that of the Lectro. But the big difference is Zaxcom gives me huge dynamic range with Never Clip on the the transmitter, internal recording with tc and full wireless remote control.