Should be able to get plenty of volume out of a well EQ'd 35watt amp.
That said, I can't see there being a huge perceived difference in volume between the 25 and 35 models. It's something like 3db difference between a 100watt Marshall and a 50watt.
What does govern volume more than wattage is efficiency. An efficient 5watt amp will kill an inefficient, more powerful amp for perceived volume. Which explains why valve amps are louder than most transistor amps of the same power rating. Valve circuits are all pretty much based on those old RCA designs. Like speakers, the V30 sounds louder than the G12-75 because it's a more efficient speaker. 101db vs 97db if memory serves.
I built an 18watter years back. Took great pains to make sure it was pretty flat in it's eq. Because I wanted to get the sounds I needed from pedals. I was playing in a cover band at the time and needed as many different sounds as songs in the setlist.
First gig with it was open air in a pub beer garden. On the back of a lorry trailer.
1000watt PA. 400watt peavey bass amp. Drummer hitting an un-mic'd Gretsch kit from the 40's (every drum was enormous). Turned my 18watter up to six. It did the job fine.
Good article on watts and decibels here
http://www.300guitars.com/articles/amplifier-loudness-watts-and-decibels/
