Can't tell,if my house is properly grounded

I second Erik's suggestion of having a tech look at it. There is the possibility that a safety cap in the receptacle is leaking ac, especially the one from N to ground. If the fuse blew with that extension cord, it's possible you reversed polarity and ac went to ground through the bad cap, especially if there is an earth line choke between the safety caps.

To be clear it made the noise prior to me trying to use the cheater plug that flipped the polarity. 

One more test.

With only the preamp connected to the power amp, TT not connected to the inputs of the preamp, unplug the preamp from the AC power.  Turn on power amp and check for the buzz/hum.

With only the preamp connected to the power amp, TT not connected to the inputs of the preamp, unplug the preamp from the AC power. Turn on power amp and check for the buzz/hum

Did this and no hum.  I'm not clear what that proves or disproves, if the preamp isn't plugged in then the ground isn't plugged in and no ground loop can occure. 

I just unplugged everything but the amp. I didn't notice before the same buzz/hum is there but bearly audible. This make me think that the noise is in the AC line. The phono preamps amplifies small signals so is it possible this bearly audible sounds becomes easily audible once it's boosted?

Just to add to the general dust cloud that the OP is removing hair over...

I had a persistent 60cps 'hmmm' that sent me on a search & destroy mission.  Went through the usual 'disconnect this/eliminate that' routine on my system to no avail.  Moved on to the ac circuit, unplugging or disconnecting everything, including the GFCI 's that were part of that line...

It persisted.  Visions of 'Reddy Kilowatt' giving me the middle finger salute.

Spouse said "Why don't you try a different circuit?"  Since I subscribe to 'the women are smarter' theory (as they've got us pretty much where they want us), I got out the extension cord...3 wire, of course.

*Poof*  Gone.  Beats me as to why.  Only operative reason may be a noisy breaker.  Since the existence of the new extension cord isn't a tragedy nor an issue I'm in no rush to buy a new breaker, BUT...

It could be that simple.  Easy enough to try, can't hurt to do so.

Good luck.  Spouse got thanks, big hug and kiss. ;)  She likes my hobby...she likes how it all sounds as well although the complexity of 'the mains' confounds her.  So the project at hand is to develop a simple 'work-around' for her to enjoy the solution as well. *S*  Marital bliss requires maintenance...;)