Threshold verse Pass Labs how can I justify significant price difference

I would listen long before I made the decision a Pass was an upgrade.

Also on your list should be Ayre and Parasound.

Ayre IMHO is a much better sounding amp than Pass, and the Parasound may be a significant upgrade just from virtue of being new, withoug the megabuck costs of the Ayre or Pass.

Of course, Bryston too should be on your list.
Above all, your your own ears, and not necessarily the price tag to guide you.
Best,
E

If you want to hear a significant difference, I would forgo the X series class AB amps and go for the XA series class A amps.  And use them for the HF/MF in the bi-amp configuration.

So as far as I can see, I should live alone my setup.
the only reason I would even look at 350.5 is  its looks and appearance.  And since there is no significant difference between S series and Pass X series there is no point for me to spend money. 
BTW I’m aware my amps aren’t the best but I like how they sound and look and I like to stick with Pass design amps.
thank you guys for your opinions 

The thresholds are very nice but the Pass amps have completely different circuitry and you will find significant differences in sound vs the threshold amps.  

I would still keep a threshold amp as a backup amp.  The perceived differences between Pass amps is an entirely different matter.  I would have to hear them in my system to judge. 

However, no contest between the still very good threshold vs Pass gear

^FWIW, I prefer the looks of the S 500 without the distracting glowing meters of the Pass Labs.

Well the blue glow was one of the issues I have with the look of Pass 

I just bought a pair of Threshold FET 10 line pre and phono pre and they are over 25 years old. I have to say they are as good as anything.. up to say $20,000.
SO I would tell the op, switching to PASS amps would be total LATERAL swap, just to spend big bucks. What you would have a newer amps with a company still in business.. But not any better sound. Now maybe if you were spending $30,000
The list on the 350.5 was $11,550 in 2013 You r current amps would probably have cost the same if they were built in 2013...
I say keep your amps and buy something else (not a new amplifier), that is actually an upgrade.

Valid point and all opinions lead me to conclusion to stop improving my setup and start enjoying it.  All doe I always wanted FAT 10 both panels so I just might get it and see if on pre amp stage I could improve situation 

Get the the Pass and find out for yourself. You can always sell it, if its not what you seek. But I am sure you will be wondering what the hell the people in this thread were talking about.

I'd hardly say there's no significant difference between Threshold products and Pass Labs! Major differences to note: Pass Labs amps use FETs instead of BJTs, they use Su-Sy feedback, they're high bias region is single ended rather than balanced. They represent the more evolved designs of a more experienced engineer. 

Unless you're using obscenely inefficient speakers, which you're not, you don't need 250+ watts. I'm listening to 91dB sensitive speakers powered by a 31 watt Pass F5. It's more power than I need. And it sounds excellent. 

So another words there is big difference between say S500 and 250.5?

It sounds like no one posting above has direct experience comparing the two, so their guess is no better than your own.  Hopefully someone who has compared the two will chime in.

I have to agree with the comment about no one has (directly) compared the two. The person saying the Pass 'has' to be better, by saying what transistor type it uses?? My comment was also no direct experience. What I  would say in general is amps are not as different to the final sound as most of the other components.!!!Sure someone might rave about what their 'new amp' did for them. But the op already has a classy amp(s). no slouch there. So would there be any difference? yeah, and two BUTS... But is the difference in the sound going to be worth it? but is it the best thing to do/buy? IMO still a no. Now if the op was having some troubles with the current amps, and was asking to 'fix or replace' Yeah replace them with a Pass.And maybe keep that in mind when the current amps do have problems.. in ten years...??

Sorry, not having read enough of this thread, I may be repeating something here. I owned a Threshold S500 and took a peek inside. It was impressive, 'for it's time', I will say that. As well built as it was, there was an issue with the channels being cooler than they should, which is a bias issue. My point here is that to resolve all the issues associated with getting the amp back to spec was not worth the effort. The search went on from there to find a suitable working replacement.

I’ve heard both. Both similar and different. Neither necessarily better. YMMV.

I think I'll stay with my current amps after all of the input you guys give me.

Yeah. They're completely different architectures with completely different gain devices and very different feedback schemes. Nelson likes to say MOSFETs are superior output devices so there's that when it comes to transistors. 

If you like what you've got, fine and dandy.