My two-bits: Pre-Marklin LGB locomotives (1985 - 2006) quality can't be beat for large scale after replacing the electronics and installing a new DCC/DC sound decoder. Marklin-produced LGB (2007 to date) are also excellent but some of their quality control at the factory is lacking. Since 2014 Marklin stopped using Massoth for LGB locomotive electronics and uses their MSD3 mfx/dcc/analog dcc sound decoders which are good but nothing compares to sounds produced by the ESU 5XL dcc sound decoders..........far superior than Massoth or Marklin-LGB. And to properly set up the ESU 5XL sound decoder, including to install a pulsed Massoth or ESU smoker, you do need to use an ESU LokProgrammer, or find an ESU dealer like myself who will set up the decoder for lights and smoker voltage, set the LGB Buhler motor type, etc.
I've never used Zimo so can't comment on its sound quality. I do have a customer, however, who sent me his three Zimo-decoder equipped LGB locomotives to replace the Zimo's and install Massoth or ESU sound decoders.....this is just one customer that's requested that. I have demos on my Website's Homepage, of ESU 5XLs I've installed into customer LGB locomotives, such as an LGB Uintah and the sounds are amazing:
LGB Trains | Old Dominion Railways | Repairs & DCC Installs | United States
I do also like Massoth and sell them and install them into customers' LGB locomotives. But the American Mogul, Uintah and Mikado sound files have chuff distortion at low speed step range........I've complained to Massoth Germany several times but they still haven't fixed them. I recently installed the US Steam Locomotive K27/K28 sound file into a Massoth LS DCC sound decoder he purchased from me and didn't find any chuff distortion, so I recommend using that steam sound file for American steamers instead of the Mogul, Uintah or Mikado files.
I'm also a Soundtraxx Tsunami2 4400 4 amp and 2200 2 amp DCC Decoder dealer. These are also excellent sounding decoders although they are generic steam, diesel, electric sounds installed by Soundtraxx. They do have an amazing 65 whistle options and 12 bell options built into the decoder that you can set with your DCC programming track. These decoders have no reed switch or volume level functions, but you can set automatic sounds such as whistle sounds when starting, stopping, same with the bell, especially helpful for analog operation. In DCC operation, you have full activation control of all the sounds. I use the smaller 2200 2 amp decoders for converting LGB Olomana and Chloe locomotives to dcc/dc sound.
The only downside with the ESU 5XL, if the locomotive is going to operate in analog mode, the reed switch function doesn't work properly on the decoder/sound file, at least for American locomotive sound files. If a customer needs that function, I then recommend using either the Massoth or LGB 55029 Retrofit MSD3 mfx/dcc/analog decoders for installation.
Lastly, please note that the Marklin/LGB factory installed MSD3 mfx/dcc/analog sound decoders have very limited sound adjustments, and you can't install a different sound file on them. The factory "locks" the decoders. However, the LGB 55029 Retrofit MSD3 mfx/dcc/analog sound decoders' are capable of installing any sound file from the Marklin Germany sound library, and there are options sounds like whistles, bells, that can be installed..........in fact you can install your own sound files. I just completely refurbished to like-new condition a customer's Marklin-produced LGB 20280 50th Anniversary Mogul Steamer used in a restaurant's overhead railway with lots of wear on it with the following: New MSD3 mfx/dcc/analog sound decoder which I loaded with the same Silverton Mogul sound file that's installed in the just produced LGB 20283 Silverton Mogul now being shipped to the USA market. Also totally rebuilt the motor block: Flushed clean, new motor, idler gears, carbon brushes, slider shoes. A four wheels sets replaced with new sets with new gears, traction tire. The existing pulsed smoker still works well. You can see a demo video of it operating and the various sounds activated on my DCC Test Stand at:
LGB LOCOMOTIVE DEMO VIDEOS | My Site
My two-bits on DCC sound decoders is concluded.