Optical Test: Cisco 2960 to ER switch
So I obtained a Cisco certified Optical Module for my 2960 and after initially getting no functionality I looked at the switch via console and saw it had disabled the Gig port I’d previously used with the other Optical module:
GigabitEthernet0/1 is down, line protocol is down (err-disabled)
I moved it to Gig port 2 and it lit and worked - plug and play! 
I then played some music and was initially quite pleased by a more open breathy sound presentation with plenty of attack and good timing - but it became very fatiguing.
Reflecting on what was bothering me I realized there was a ultra-HF brightness that previously I associated with HF oscillation and instability; I perceive this a pressure on my ears similar to when you make a quick descent in a plane - I find I’m swallowing to clear it…to the short version I revered back to Electrical and the problem disappeared.
I then reflected that perhaps the Optical connected ER switch needed to have its Earth tab used as it was no longer getting the same earth - made-up an earth lead and that solved the problem - all HF instability gone.
Listening tests:
Over extended listening tests I found the Optical had strengths and weaknesses in my system.
The strengths was an easy breathy clarity and precision across extreme LF to HF - an even-handedness I liked. But it was not drawing me into the performance like I had with the Electrical Ethernet connection between the switches - it seemed more pushed at me and had something missing.
Back to Electrical and it became clear that I was getting a lot more detail overall now, even if the Optical had done some things better.
The Electrical Ethernet portrays the whole fabric and textures in the music as it plays without disturbance - it is just there and you can listen into it - in fact it draws you into what is happening.
By comparison the Optical sounded superficial ‘on the surface’ with poor depth of detail. It did simple music well but more complex music with many things happening it drew you to the loudest instrument at any time and the rest became indistinct to not there.
Perhaps the GigE Optical module sub-system is noisier inside the ER switch. I associate the ‘breathing’ of detail to the music with interaction with non-linearity somewhere, but exactly how it is manifesting I’ve no clear idea.
Also this ‘breathing’ distortion is one thing I personally dislike and navigated toward Active system as that tends to not do that. It very well may not be at all apparent in another system to another person - perhaps.
I’m describing what it appears as on my system for me. The effect is ‘moderate’ but once you hear it you do not choose that as the way ahead.
…I’m still experimenting - I want the Optical to musically work as it will make for a tidier solution here - and this may be very specific to how it works with an ER switch - it may not be general.
…also the earthing line changes the sound and is another ‘better and worse’ thing - I had to eventually remove it when back to Electrical Ethernet use as it added a lumpy bass coloration that went with it gone.
More experimentation ahead here - I have some ideas to try to do that better.
Essentially never presume things are all the same if you are really interested in getting the best from what you have. The alternative is a £5 Ethernet cable vs £30 for Optical connection if you also include the modules each end. It is not a lot in context of my system.
I will continue to play - glad I got it working! 
DB.
It is appreciated 
,.but it was actually Nick.Lees,who is from the UK,.who first reacted strongly and wrote about this.
But now I have more important things to tell you about,.I’m getting the first impressions of the new Chord switch 
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