I wanted to know your opinions about if I should go for the B4, B5 or B6.
B4 being 0.11 cubic feet, 0.32 cuft for the B5 and 0.48 cuft for the B6. Same tweeter in each speaker, and respectively 4", 5 1/4" and 6 1/2" Woofer. Their Low Frequency cutoff is respectively at 55Hz, 50Hz and 40Hz. Their crossover is 1800Hz for B4 and 2200Hz for B5 and B6.
I copied specs (at the bottom of this post) for convenience.
1) First, consider the low frequency question:
Forget the Lf cutoff (-10dB is not something you actually want in what you are listening to). The low end of the Frequency Range spec at ±1.5dB is a much better indicator. Also, for the PSB speakers, the Frequency Range spec at ±3dB indicates that none of these speakers fall off the cliff too abruptly below the ±1.5dB specification.
I believe the THX standard is to crossover at 80Hz and from what I have seen it seems most people end up setting their crossover between 80Hz and 100Hz. Admittedly, if you had some ($$$) B&W 802D's for your mains, it would probably make sense to take the crossover lower. But you have a good subwoofer and spending money on full range speakers with better sound than your SVS in the sub 100Hz range is not cost effective.
Go to a speaker dealer in your area and have him set the crossover to 80-90Hz (no sub) and compare between the speakers you are interested in (be sure to match the speaker sound levels). Try some larger full range speakers, as well (again with the same 80-90Hz crossover).
From this, you can evaluate with your own ears how important the extra size or number of mid-bass drivers would be to your 2.1 system.
Personally, I have a pair of EMP 41-SE/Be speakers with a 4" mid-bass driver and am extremely happy with the low end and how they blend with my (bottom-of-the-line) Martin Logan Dynamo sub with a 100Hz crossover. I believe your sub should do as well or better.
2) Secondly, and perhaps most importantly, you said:
Well, I'm not that happy*, because I want to build a simple 2.1 system around that monster, it seems to be difficult to get powerful enough loudspeakers to match it, so I narrowed my choice to the PSB new image line.
By powerful, I'd assume you mean loud? Why did you chose the PSB Image?
The sensitivity specifications show a big difference in the output between the B4 and the B6, and this is probably the main reason you should be considering the B6 over the B4. Also, consider the recommended power spec. The B6 not only plays louder per watt of input, but it can also handle twice the power (as can the B5).
So while I would consider the B4 from a blending standpoint, I would rule it out from a "powerful speaker" standpoint. If you plan to work it hard in a largish room, It probably cannot keep up! The B5 may do as well as the B6, but it depends on
how loud and
how large a room.
Again, if you have a dealer where you can listen, you should. You can often save money by establishing a less expensive unit will fit your needs, but more importantly, if you don't listen, you'll always wonder "what-if".
It would be useful if PSB had maximum output (or max SPL) data. I bet if you contacted them, they would provide.
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Image B4, B5, & B6 Bookshelf respectively:
Frequency Range
On Axis @ 0° ±1.5dB 70-20,000Hz 70-20,000Hz 59-20,000Hz
On Axis @ 0° ±3dB 60-23,000Hz 50-23,000Hz 45-23,000Hz
Off Axis @ 30° ±3dB 70-10,000Hz 70-10,000Hz 59-10,000Hz
Lf Cutoff -10dB 55Hz 50Hz 40Hz
Sensitivity
1w (2.83V) @ 1m, IEC-filtered Pink Noise, C-weighted
Anechoic Chamber 85dB 87dB 89dB
Typical Listening Room 87dB 89dB 90dB
Impedence
Nominal 6 Ohms 6 Ohms 6 Ohms
Minimum 4 Ohms 4 Ohms 4 Ohms
Input Power
RMS, Clipping < 10% of the Time
Recommended 10 - 80 Watts 20 - 150 Watts 20 - 150 Watts
Program 60 Watts 150 Watts 150 Watts