What Power Amplifier Should I Buy?

@kevemaher

Your Bryston 2.5B cubed specs show:

Power Output (per channel): 135W @ 8Ω | 180W @ 4Ω

However, Stereophile F208 review here suggests you need more current:

…. F208 will need to be driven by a good 4 ohm-rated amplifier…..

and that there was plenty of bass. Therefore a more powerful amp with adequate current is likely better than adding subs

Generally, a speaker rated at 8 ohms only hints at being average to easy to drive, while a 4 ohm is harder. But a further look into the frequency vs impedance/phase test/chart of the F208 reveals it needs more power

@kennyc 

Thanks for reminding me about this. I've seen the impedance plot on ASR. There is a dip to 3.7 ohms at about 100 Hz and 3 KHz.

I also measured the voltage into the speakers while I had the SPL at the listening position using pink noise. I found that I needed about 3 V, which is about 1 W for 8 ohms and 2W for 4 ohms.

I concluded that I have enough headroom to allow for a little higher SPL and for the low impedance at the above frequencies (100 Hz goes to my self-powered sub).

@kennyc 

Just looked at Bryston's published spec. It is rated at 180 W into 4 ohms. So it is rated at 4 ohms. However, I was cautioned that if the impedance was below 4 ohms for a large part of the spectrum, especially in the midrange, there could be problems.

I have never seen any of the overload lights turn red, although maybe that doesn't mean much.

I really haven't experienced anything audible or measured any parameter with REW that would indicate problems.

Am I thinking properly here?

Both Stereophile and ASR test show plenty of bass down to 30hz. Your 14x19x8h room looks fine, but the other impedance show the F208 is harder than average to drive. Conclusion is that you don’t have enough power. Also, if one defines “average” needed power to run most speakers, seems to be in the 150-300watt at 8ohms range, somewhat doubling down to 4ohms.

If you love your amp, I suppose you could try to fill in the missing bass, but my preference would be to get the bass from the speaker since it’s matched to the other transducers (mid treble).

Maybe you can test by borrowing a more powerful 4ohm amp?

Two subs increases bass headroom by 6 dB, as well as significantly reduces peaks and nulls from room modes. You can’t EQ out a null, which can be 15 dB or more deep, and worse, the nulls are in different parts of the room, so even measuring them is tricky. Two (or more!) subs fill in for each other, since they are separated from each by many feet (preferably as far apart as possible).

A 6 dB increase in headroom is nothing to be sneezed at. 3 dB comes from having twice the power (two plate amps instead of one) and the in-phase in-room summing doubles speaker efficiency, so the net gain in headroom is 3+3 dB, a fourfold power gain. That’s equivalent to either getting 6 dB more efficient speakers (think horns) or 4X the power amp, assuming perfect loudspeakers with no power compression.

In reality, speakers with efficiencies in the 87 to 90 dB/meter range typically experience power compression with amplifiers more powerful than 100 watts, so a 500-watt Class D power amp is not necessarily the answer ... any loudspeaker will sound audibly "squashed" or distorted if too much goes in. Remember, a 92 dB/meter/watt speaker is only 1% efficient, with the other 99% of those expensive audiophile watts doing nothing more than heating the voice coils.