Class A vs AB

Oh, one bit of advice I try to give when I read about other's enjoying Fritz.  They do sound better in rooms with at least a little damping. Honestly at shows it's kind of amazing how good they sound, because well, hotel room, but he always travels with a few panels, which dampen the high frequencies enough to let the bass bloom. 

OP

I have 2 Pass labs Amps. an XA-25 and X150,5. Speakers are ATC SCM and Falcon Gold Badge. in my opinion,

- the Xa-25 cannot make the ATC speakers sing, but the X150.5 brought my ATC to a different level. a level i never thought possible

- the XA-25 has synergy with Falcon so much, that XA150.5 cannot match.

in the end, the class A only Xa-25 and Class A, AB X150.5 always stays in class A and hardly hits AB and still cannot make the Falcons sing as compared to the XA-25

IN the end, it all about synergy and that Synergy is how the individual listener perceived the music. 

You mentioned that one is listening to about 90DB. at that level, ROOM acoustics is already playing a BIG role. If you have done Good Work with ROOM acoustics, YOU will Definitely hear a difference (depending on music) and may account for class power difference. again, synergy is all the more important factor here. 

Here is the balance the way I see it.  "Current" is what pushes the woofers, and this is what makes bass.  For super efficient speakers, the highs and mids are loud without much current which can hinder the movement of the woofer cones, thus giving you less bass.

@hjdca  It does not work this way. You do need more power (not current, since Voltage must be present for current to be present and the two together are power) to do bass frequencies since there is so much more energy there. But what you're saying about high efficiency speakers is simply false. 

   I am considering a pair or Arandal 1528's (84 db sensitivity I believe, 4 OHM) I understand each speaker is unique to itself. At 90 db listening level, give or take, with the 1528, other than running most likely in class AB, at higher volume setting, can u even identify the difference in AB vrs A or plainly too many variables. 

@robshaw at 84 dB and also 4 Ohms, the speaker's efficiency is only 81dB. The Fritz is 87dB and 8 Ohms so its efficiency is also 87dB (since the sensitivity measurement is 2.83Volts at one meter, which into 8 Ohm is 1 Watt but into 4 Ohms is 2 Watts). In addition, with less efficient speakers there is a thing called 'thermal compression' that causes a loss of dynamic contrast. 

I think you'll find yourself missing the Fritz in short order!

You cannot hear the transition between A and AB. In fact you've been hearing it all along (since musical peaks take more power than most people think they do) and not been bothered by it now. So no worries in that regard.

However, all amps make more distortion into lower impedances and that distortion, especially when you're asking 4 times as much power, is going to be audible as being harsher, less detailed and likely brighter. That is because the distortion increase includes higher ordered harmonics, which the ear senses as brightness and harshness (this is because the ear uses harmonics to sense tonality which is why you can tell the difference between an acoustic and electric guitar). 

If you want to move forward, pay attention to impedance and the sensitivity of the speaker candidate. Higher sensitivity is better and 8 Ohms will sound better with all amps if other variables with the speaker are rendered equal.  

Thanks all very much. Joseph audio pulsar has been in my sights.(used) Some  people prefer the originals rather than the graphene. I have been intrigued by the Marten Oscars, and the Acora mrb-1. The ATC's always caught my eye, albeit harder to drive. I guess my pass X-250.8 should be fine. Not sure which model would be the best in the room in question. Always been curious about the SCM 19, but, thought it would be similar to the FRITZ. Was thinking about the SCM 50 (used) However, not sure whether it would be too much fro my smallish room? Thoughts??  Thanks Robert

@robshaw 

I have a smallish room too and if you control it properly a floor stander will work fine. I recommend auditioning Rockport speakers. They are superb.  My  Avior ii speakers sound fantastic in my room. Good luck!