As you can see in datasheet 220V, -6.8V, at 20k load is valid operating point.... but distortion is high (up to 7%), anode current is low (6.5-9mA). The most linear part of curves about 100V Va.
I have ECC88 tubes, but only use more robust E88CC.
According to my experiences (I have a few dozens 88CC variants -Philips, Telefunken, Siemens, Tesla, Tungsram, JJ, russian) at higher Va and low Ia the sound would be thinner, metallic (I think this is due to the growing 3th harmonic distortion).
I'm in love Siemens CCa "sound", fortunately I have few tubes with good condition.
