Ethernet to SPF converters. What's your experience?

I have had experiences with FMC devices.

But first - the first response to the OP was about clocks.  I am interested in this subject.  It looks like @sls883 has a Gustard DDC.  I am curious to know if @sls883 is using the I2S input on the DAC.  It is my understanding that a good way to use an external clock in this situation is to connect the external clock to the DDC and then connect the DDC to the DAC with an HDMI cable and the clock signal is 'transmitted' through the HDMI cable to the DAC.  I don't understand " I have the clock connected to the N18pro, the U18 ddc, and the X-30 dac". Does the clock have 3 outputs?

Now on to FMCs:  My experience was initially with a Sonore microRendu which has ethernet in and usb out.  I used two "cheap" FMCs in series to provide galvanic isolation (I think I'm using the correct term).  I used the wall wart power supply in the "upstream" FMC and a Sonore LPS in the FMC closer to the microRendu.  I used a cheap fiber cable.  For me there was an improvement - more "relaxed". less digital "glare".  Not a big investment, the most expensive item was the LPS.

The next step was to replace the cheap "down stream" FMC with an upgrade one from Sonore, which also included an upgraded SFP module.  Similar improvement as above, but maybe more subtle.

Next I replaced the microRendu with an opticalRendu which has fiber in and usb out.  This eliminated the upstream FMC.  This was a big improvement, not inexpensive.

Then, over at the Audiophile Style forum I read a long thread about using a Direct Attach Cable (also unfortunately referred to as a "DAC") which replaces the fiber cable and ELIMINATES the galvanic isolation!  I bought an inexpensive DAC and I like it better!  So I'm currently streaming through an FMC but I'm not using any fiber cable in the chain.  Pretty crazy.... 

as @audphile1 has said, you just need to try this stuff and see what sounds better to you, in your room with your system.  It seems that 100 hours of "break in" is a good idea.  And always buy stuff that is returnable. 

I think cheaper is better, but where the AC adapter is matters.  If it's near your equipment or sharing an AC line, keep it on the "dirty" side of a power conditioner and consider an aftermarket supply like these.  They won't make the network connection better but they will reduce the amount of noise in your AC line. 

Thank you for all of the suggestions. 

I’m very familiar with the sound when connected directly from router to streamer via ethernet.  I’ve had it that way for a while. I bought the switch a couple of weeks ago.  So far, I like the sound with it.  I’ve only spent a few hours with the ethernet to fiber converter.  Jury is still out on that.  The little wall wart is plugged into the same power conditioner as my other gear, so that might be a problem.  I’ll look into that.

@boulder_bob Yes, the LHY ock-2 clock has three outputs for square wave and three for sine wave (I’m using square wave).  "more relaxed" is kinda what I’m hearing with the converter, but I’m not sure if I prefer that.  I need to do more listening. 

I have two systems on fiber. Both use switches with built in SFP fiber ports and both my streamers accept SFP fiber inputs. Definitely a lower noise floor. Before all that I tried those 2-piece ethernet to fiber converters, but too many connections nearly erased all the noise stripping that fiber does. So I bought the switches and streamers that had them natively. Much better. Lower the noise floor, blacker background = better more detailed audio. Btw, the AOC SFP modules are the best. 

Yes definitely isolate the wallwart from audio. 

I’m running Nucleus One and a basic Trendnet Switch. Nucleus is plugged into a dedicated circuit 1, system (DAC and integrated) are on dedicated circuit 2. Switch is plugged into a completely different circuit. 
The Trendnet switch, to my surprise, smoothed out digital glare when used with Nucleus. I’m also running WireWorld Starlight 10 Twinax Ethernet cables from eero to switch and from switch to Nucleus. That was a very good improvement as well. The WW Ethernet cables are really good.