Instruction: Answer the given question by extracting information from the supporting document.

Input:
Question: What are the primary differences between Model 1540 and Model 1600? I have L/R Back & L/R Surround and assume I would have to buy two units regardless
Document: Snap...  Crackle.... Pop!!! I have the 1540 model, the 400 and 480 models and have the same complaint with all... The units have auto shut off. What happens if you are watching a movie or TV and there is no soundtrack for the surround sound speakers for 2 or so minutes the units shut off, goes into standby.
Sad because the sound is excellent when they are working. A simple on-off switch on the units would have eliminated the problem. My previous set up had on-off switches and I experienced no problems, the unit stayed activated till you turned them off... It is an annoyance that ruins the surround sound experience and is due to poor engineering design... I contacted the mfg. and they responded by saying it was a SIGNAL PROBLEM???
Only other issue is that it ships to me in New Zealand with a European plug top power supply - but I'm used to that from all other international suppliers anyway.Great product - if I had a similar requirement I would buy more of these.
Once the signal is absence for the specified time the system goes to sleep/standby; when the signal reappears the units energize and cause the pop. Do away with the auto shut off and install on off switches so the units stay energized regardless of signal and you eliminate snap, crackle, and pop. As i said my previous system operated that way and I never had the issue.
This wireless kit does work with my system, and the sound is good. (I'm using them with my back surround sound speakers.) However, I should have paid more attention to the reviews that mention popping sounds. I'm not sure if it's when they go into standby mode or when they come out of it, but there is a loud, startle-inducing pop every time.
I live in a old steel house where three is almost no way to run wires in the place and not have it look bad. So I bought this Amphony wireless system to get my surround sound. Works great, I can adjust the level of the back speakers, use them as satellite speakers and set them up in another room, gone about 30 feet with no delay or loss of signal.
I needed a solution for my rear home theater speakers. I did not want to go for a 2.4 GHz kit because of the possible interference. Also, I did not want to spend the money for the Amphony 1600 so I compromised and got this kit.Performance is more than OK. Great sound. Range is a little limited but is is more than enough for my studio. Would recommend to anybody.
watching a movie on TV is even more annoying. I now unplug the receiver/amps if I am watching either of those. I do not recommend these units on that basis unless you are willing to tolerate the annoyance.
This 1540 kit does exactly what it promises. It eliminates the cable to the rear speakers though you will need power to power the receiver. If you don't want nasty speaker wires then this is a good choice. Wattage is quite adequate.
This kit only comes with one wireless unit so you have to run some wiring from one speaker to the other which sort of defeats the purpose of wireless.Overall I'm very satisfied with this kit. I would recommend it to anybody looking to make their rear speakers wireless.
Answer: You can't use 2 kits in the same room. In your case you need to purchase the Model 400 transmitter and Model 480 wireless receivers.