This question is aimed to TRUE Elec Engineers, not fuse or wire directionality believers.

Let me try & respond to some of the comments made recently.

- First, a fuse will NOT protect a speaker from a DC fault.
  If and when a fault occurs, the speaker wire, especially the tweeter wire
  will melt and catch fire WELL BEFORE a fuse has time to act.
  Therefore your speaker will be destroyed, saving the fuse.  The ONLY 
  sure way to ensure that the fuse will blow before the speaker is to use
  a crowbar circuit.  I dont like the idea of that.

- Secondly, normal relays, regardless of whether they can carry
  HUNDREDS of AC amps will NOT be able to break a DC circuit,
  In a fault condition, The DC will arc and continue destroying
  the speaker and the relay.  At most, if one is lucky, the relay may save
  the speaker but it itself will most definitely be destroyed.  Look at
  relay specs CAREFULLY and note that MOST are rated at no more
  than 30 Volts DC.  I have +/- 90 Volt rails.

- The amp I am building is high power BIPOLAR transistor based
  so in case of a fault, it is more likely to fail as a short than an open
  circuit and hence more likely to destroy speakers as compared
  to a MOSFET output stage one.  Hence the need for a reliable
  speaker protector.
  
- Most commercial amps use a relay in the output MOSTLY to avoid
  thumps when first powered on.  Altho they MAY protect the 
  speakers in DC fault cases, their intention is the former not the
  latter.

- Thirdly, SSRs made with back to back MOSFETS driven down
   to a few milliohms will make a PERFECT wire, nobody (except
   the superhumans on some of these threads on this site) will hear
   the existence.

- As for the NASA fuses/relays, I know about them, they are
  encapsulated in nitrogen under pressure so that they MAY be
  able to break DC but they cost in excess of $200 or so for a
  15 amp relay.

- The control circuit I intend to use is one by Rod Elliott and here it
  is:   http://sound.whsites.net/project33.htm 

Thanks

cakyol
- As for the NASA fuses/relays, I know about them, they are
encapsulated in nitrogen under pressure so that they MAY be
able to break DC but they cost in excess of $200 or so for a
15 amp relay.

>>>>Nice that you know all about them but apparently what you don’t know is that advanced audiophile fuses can oft exceed $149 and sometimes even exceed $200. Maybe NASA needs to step up their game. Audiophiles have two or maybe three conflicting requirements, cost, sound quality and protection.

This is more like it:

https://www.mouser.com/ProductDetail/Infineon-Technologies/IPB017N10N5LFATMA1?qs=sGAEpiMZZMshyDBzk1%...

100V, 180 amps, 1.5mohms (almost lower than the speaker wire itself), about 80 nanosecond turn off time (increase that becoz of inductive speaker load) and only about 8 bucks a piece.

In theory, it looks perfect, the only issue is with switching MOSFETS, they dont unfortunately specify the frequency response....

The MOSFETs will be hard ON all the time- you don't have to worry about bandwidth, as the resulting circuit will be good from DC to well past 100KHz. They have some capacitance associated, but that will be negligible unless your amp is really high output impedance!

As long as the MOSFETS stay hard on between 20 - 20khz it will be fine :-)