It seems unlikely that the HD was the cause of the loop biting the dust.  Having said that:   it would be interesting to hear what comes back from the repair guy as to their suspected cause too.

Sorry mate ... sucks!

There are amp schematics here if your repair guys need them -->  https://docs.google.com/viewer?a=v&q=cache:SdXt-TLDTioJ:www.traynoramps.com/downloads/servman/smycv40.pdf+Traynor+YCV40+manual&hl=en&gl=uk&pid=bl&srcid=ADGEESjcgAssMooiIuH8QvPj79OpsoWQa3z-w9BV8OsFJS_CTy8lPzsk7OO3bMNz8m80m7DlK8AQYpiNCJhTv_H-iOS9Z8dfHvzCKeaKTipNWa1EazAjXtfGCqvDMW4MUXUdd1YynGJX&sig=AHIEtbSmK4MTvqjzlQiC4BSkiuBXSaY3bw

Apparently they can be downloaded from the Trayor site as a PDF but that seems to be down.  Hence the google link.

It looks like the tubes in the amp are used for pre-amp, phase splitter and power tubes only so yeah, the loop is all solid state.

The manual describes the loop send as -10dBV (standard instrument level) but describes the return as EFX/Line.  The 'line' bit suggests that a line level return should be fine (+4dBu).

On adamf's point.  I have been digging around too cause I have been interested to see whether jacking a line-level connection into a power amp is always 'safe'.  So far I have not seen any evidence that would suggest that running a line level (+4dBu) into an instrument level loop/power amp return can cause any damage other than creating unwanted distortion.  This is what I would expect too.   The +4dBu is about 3-4 times the voltage of -10dBV but you are still talking in the range from 0-2V which is very low.

From a power amp perspective:  using 4CM is identical to using a 'Stack/Combo Power Amp' type connection with the POD jacking straight into the power amp so 4CM is not inherently any worse than this.

I guess it is not beyond the realms of possibility that Jacking a line level connection into an instrument level loop return might fry a very sensitive component or circuit but in practice people do it all the time, not just here but in recording situations too where people constantly inadvertently mismatch the levels.  I just don't see that it can be a major issue.   You will wreck your tone but I think it is highly unlikely that you will wreck your gear.

The Traynor guy sounds much more plausible.  There are notes at the end of the service manual link I posted showing some kind of mod relating to the standby switch so it sounds like there is a known issue.

Sounds promising.

I would strongly recommend getting the Standby repaired.  It's a good idea to warm up tubes before sending juice to them.  Otherwise it'll cost you in the long run with buying tubes more frequently.  Groove tubes can be really nice depending on which ones you buy.  I've found that Groove Tubes LS is the best, M is good, and don't buy R.

Wow, didn't even notice the year.