your obviously going to have different power outputs with the different tubes. so if power is a concern that may be the driver of your choice. all those tubes sound great in a good amp with slight differences. biggest being your going to be comparing SET 300b etc and Push Pull El34. one other option you may consider if you need more power in an SET is Parallel SET this has two tubes doing the amplification in a parallel configuration in a 300b this would essentially double the output power to 15wch roughly. in a parallel amp both tubes are SET and both do the full wave ( so not push pull with cross over distortion) they are summed at the output so doubling the power. I like every tube you mentioned all have strong points and week points Push Pull will always feel more powerful as they usually are. SET will tend to have a more romantic mid range but suffer at the freq extremes. ( note good output transformers can eliminate this some what).
I'm actually surprised your getting satisfying sound from your 8wch SET driving those speakers at 88db they are not efficient.
one thing to consider is what makes a SET tube amp great, 1. stable clean noise free power supply 2. quality output transformers. the first gives you low noise floor (very important in high efficient speakers), drive and power reserves. the second (arguably the more important) determines greatly the freq response mostly at the freq extremes (poor out put transformers tend to roll off the highs and lows more the quality ones). the rest is minor with tubes somewhere in there close to the top as well. the transformers are the most expensive components in a tube amp. power transformer, output transformer and choke if installed.