Hello Mattjis,

I'm sending you one of my favourite schemes of tube amplifier.
This is Brimar VA12 20W power amplifier.
The performance is:

Distortion:       less than 0.1% for 20W 1kHz
Sensitivity:      130mV for 20 watts output
Bandwidth:        Flat to 0.1 dB between 30Hz and 20kHz at 20 watts
Feedback:         36dB overall feedback
Hum and Noice:    80 dB below 20W with input shotr circuited.

If you want i can send you some descriptions of this circuit.

Some rules for construction:
The chassis is made from 16 s.w.g. alumimium sheet.
The ECC807 input valve must be mounted sa far as possible from the mains
transformer.
The amplifier has only one earth point, this being in the input stage.
Valve heaters are connected in parallel, with the ECC807 input valve
being connected directly to the transformer heater winding.
For the input stage use high-stability cracked carbon resistors.

Output transformer used in VA12 :

Type: Ultra Linear Push Pull
Rating: 30W
Primary Load: 6600 Ohms Anode To Anode
Primary Screen Grid Tabs: 43% of 6600 Ohms
Primary Inductance: 200H
Primary/Secondary Leakage Inductance: 20mH

The amplifier may be driven with a suitable pre-amplifier such as the
Brimar V10. If you want i can send its circuit too.

Performance: Warm and vivid sound with a little bit agressive discants
at high levels. Good bass performance. 
I've tried it with larger transformer (about 50W) with 6600 Ohms primary
load and 130H inductance. The sound is more easy going, with more
expressive details, than the original transformer. 

I'll prepare an easy sequency for building of output transformer with 
examples and will send it.

Bye,
Rutcho