The Retro LS-3.5 speakers are what we recommend.
Not just because we designed them from the ground-up but their 90dB/2.83V sensitivity makes them a perfect match for a tube/valve amplifier such as the Stereo 50.
The 90dB/2.83V is for approximate 30m^2 / 300ft^2 rooms. For larger rooms increase the sensitivity requirement by 3dB for each doubling of the floor area, so for 120m sqm / 1300sq ft, the minimum sensitivity should be 96dB/2.83V.
This allows around 100dB undistorted peaks.
The common peak-to-average ratio of music (classical, tutti/crescendo) is around 14dB, so 100dB peaks allows an 86dB average, which is the normal SPL in a mid-hall seat in many concert halls.
Obviously this formula is an approximation and if one likes to listen to music very loud, the efficiency required may be even higher.
Otherwise we recommend a minimum of 90dB/2.83V sensitivity (real – independent 3rd party tests preferred) and a nominal impedance of 4 – 16 Ohm with a minimum impedance no lower than 3 Ohm.
The 90dB/2.83V Sensitivity is the key number to bear in mind when selecting a speaker to match with the Stereo 50.
Question: What about mainstream bookshelf speakers?
Answer: Most mainstream bookshelf speakers were designed for 100W> amplifiers.
Hence despite their diminutive size, they are still not suitable for the Stereo 50. Typically, such speakers range in actual sensitivity from 82dB/2.83V to 85dB/2.83V. This makes them a less than ideal match with a tube/valve amplifier such as the Stereo 50.
