The speaker

To describe speakers is a difficult issue. Though the speaker plays a big part in the sound of an amplifier.
Basically, you can distinguish speakers in types with Alnico magnet and with CERAMIC magnet. Widespread are 1x8, 2x8, 1x10, 1x12,2x10, 4x10 but also mixtures. A larger speaker usually has a higher efficiency, which brings our small amps forward. At home, an 8" is great, little feedback and quiet! For the stage they say that you should have 2x10 or 1x12. For real compression you need at least 12"+10" and 4x10". On the recordings you do not hear the difference between the speaker sizes as strong as you do live in a small room.
Saying Alnico is better for Harp is religion - it always depends on the individual type. The size of the magnet is not a criterion, such as Gussow says. At 12" feedback is more of a problem than with 10" but new speakers sound (played) not worse than very old ones.

Gladly tinkering around with the 6V6 power amp and you want to get the maximum power from the transformers for a 6L6 - no audible volume gain. More profitable is the choice of speakers. We start from a 5W Champ who has a speaker with 92dB. Double power brings about 3dB more. A 101dB Speaker would theoretically "make" 40W out of the 5W.

8 inch

A good 8" already brings a very nice sound, as long as it does not have to be loud. For a small, portable amplifier an appropriate speaker size. Combinations of several 8" are not so great, I would prefer a 10" then.
The italian Jensen are easy to get. The C8R sounds very neat, the P8R sounds more round, a nice overdrive but little amount of bass. The blue "Mod" is not recommended.
A better example would be the SIG8 by Weber as either Alnico or Ceramic, but they become rare. At the same level, there are the Vintage P8R copies by VOR (Voice Of Rock), but they have to be imported. Available in Germany, relatively low-priced and sounding very good .... the WGS G8C.

10 inch