The question really is, which speaker, Sonantina or Victory would go better with your Graf 20 watt OTL. Without hearing this exact combination, comments as to which speaker is better without disclosing which amp(s) was used are of little value (ie., the Sonantinas would probably sound better if you used a 100 watt solid state or tubed amp and the Victorys would sound better if you used a 3.5 watt SET.)
I'm currently running a pair of Victorys with a Gaincard (25 watts) with satisfying results. I have tried a 3.5 watt SET with them and they played pretty loud without clipping. The midrange is as smooth as silk. The ribbon tweeter is extended without getting hard or hot. Superb on acoustic music, but can also rock.
The Victory was designed for use with low powered amps, especially SETs. On paper, The Sonantinas will need more the double the amplifier output to keep up with the Victorys at the same volume level (and that is not counting for the easier load the Victorys present, 14ohm vs. 8 ohm). A fairer comparison would be to compare the Coincident Super Eclipse (there is a recent review in the Absolute Sound) with the Sonantinas. If you are at all considering trying SETs in the future, buy the Victorys.
Generally speaking, OTLs have a tough time with loads that drop below 8 ohms. I do not know how far down the Sonantinas drop in impedence, but the Victorys is spec'd out not to drop below 10 or go above 18 ohms. This presents an easier load for your Graf, and you will be able to play the Victorys much louder and with greater ease.
One final note. If you do get the Victorys be very patient. They need a long time to break in. About a 1000 hours before they are fully integrated. Before break in there is a discontinuity between the bass and midrange.
I'm currently running a pair of Victorys with a Gaincard (25 watts) with satisfying results. I have tried a 3.5 watt SET with them and they played pretty loud without clipping. The midrange is as smooth as silk. The ribbon tweeter is extended without getting hard or hot. Superb on acoustic music, but can also rock.
The Victory was designed for use with low powered amps, especially SETs. On paper, The Sonantinas will need more the double the amplifier output to keep up with the Victorys at the same volume level (and that is not counting for the easier load the Victorys present, 14ohm vs. 8 ohm). A fairer comparison would be to compare the Coincident Super Eclipse (there is a recent review in the Absolute Sound) with the Sonantinas. If you are at all considering trying SETs in the future, buy the Victorys.
Generally speaking, OTLs have a tough time with loads that drop below 8 ohms. I do not know how far down the Sonantinas drop in impedence, but the Victorys is spec'd out not to drop below 10 or go above 18 ohms. This presents an easier load for your Graf, and you will be able to play the Victorys much louder and with greater ease.
One final note. If you do get the Victorys be very patient. They need a long time to break in. About a 1000 hours before they are fully integrated. Before break in there is a discontinuity between the bass and midrange.