The title says it all. There is an audio myth that large amplifiers are needed to get good bass.
Is this true?
[...].
Short answer: Ideally would be to not get over 50% of max. recommended power of your amplifier when doing critical listening to your preferred songs (max. power across 20-20000Hz for your speakers impedance with low THD). That's more like a "safety" measure to ensure us that the amplifier will provide a low THD and moderate heat dissipation on top of the amp. Also, my speakers are having a 3...20 Ohms impedance, the 3 Ohms being on the low-end (20...30Hz), so indeed lower octaves are much more difficult to be handled by amplifiers. Like solderdude said, active speakers might help in delivering a cleaner audio output.
It's like buying a car, will you choose one having top speed of 170Km/h or the one able to get you up to 200Km/h? Even if you know you'll never pass over 150Km/h on the freeway you'll most likely get the second car, because it's able to run at 150Km/h without struggling and without forcing the engine too much.
Long answer:
http://www.acousticfrontiers.com/2013314thx-reference-level/ explains here that to be able to "feel" correctly the scenes from the movies (also to hear the entire dynamic and soundstage of the music we listen) we need to have an audio system able to "sing" at an average output level as loud as 85dB at listener's position and also leaving place for a 20dB more dynamic; that means an audio system needs to be able to deliver 105dB peaks SPL at listener's position.
I'd like to relate 2x100W RMS with loudness:
- For my Pioneer speakers having 83dB/W/1m, 100W RMS means 103dB SPL @1m; my listening position is 3m away, so 9dB get down (3dB/meter) and remaining loudness would be 94dB SPL/speaker. Counting the 2nd speaker will add 6dB into equation, hence total SPL would be
100dB at listener's position (lower than 105dB, so not compatible with THX standards unless moving about 1.5 meters closer to speakers).
- For my Canton speakers with 90dB/W/1m the same 100W RMS means 110dB SPL @1m; at 3m away that would be 101dB SPL/speaker at listener's position. Counting the 2nd speaker will add 6dB into equation, hence total SPL would be
107dB at listener's position (seems to meet the THX recommendations).
On the above link there's a similar calculation that could be useful; you can see there that THX-certified could mean output power up to 300W for both channels if listener's position is at 11 feet:
Seems that in my living room I need an amplifier able to deliver at least 200 Watts totally (100 Watts RMS/channel) with very low distortions (<0.1% across audio range would suffice), so I can enjoy THX requirement at 3 meters away from the speakers.
Also, my sub does not meet THX specs of 115dB SPL at listening position, for frequencies below 40Hz (between 20-31.5 Hz the average is 113.0 dB), based on
https://www.soundandvision.com/content/review-svs-pb-1000-subwoofer-measurements, so I need to get a second sub or to swap it with a bigger one (which I'll not be doing it for sure):
I do believe that having an audio system able to deliver 105dB SPL at listener's position, or even more than 105dB, it's a warranty that on normal listening levels distortions will be insignificant and there's enough dynamic and headroom left there for critical listening sessions.
Note: The above images are direct links to
http://www.acousticfrontiers.com/2013314thx-reference-level/.
Objectively there should be a very significant difference in sound...the big amp should sound better. But... we do not hear objectively except in blind comparisons. So, if you hate the big amp and really like the Hifiberry (perhaps you do or you would have tried it already) you may not hear a difference...or may think the HifiB sounds better. That's fine...use the one that pleases you most. 
