Plate amp vs. new sub vs. pro amp

So I am getting interested in building a sub. I have a PSW-505 and it is good enough but I would like to get more at some point. I know there is a lot of work that goes into building a quality speaker but for now i am trying to figure out the best amp for a project. my confusion is watts vs. price and what has the watts in these different amps really is. I relise watts are a mesure ment but still subjective to some point based on how they are measured by the compony. so My psw-505 price (189 dollars ) is a 475w peak 300w RMS i cant figure out how manny ohms that is measured at is it 4 or 8. the plate amps I see on parts express are rated at 300-500w RMS at 4 ohms priced 170-240$ . if the PSW-505 is at 4 ohms then the plate amps are way better deal if it is at 8 ohms would it be more cost effective to get cannibalise my PSW vs buying a plate amp.

A Pro amp would be great but I am not sure how there watts are measured is the Behringer NU1000 1000w at 4 oms bridged RMS or peak. I like this option but dont know how big to go and what the cost effectiveness is. If the pro amp is RMS it takes the cake. so i am just a bit confused and how crappy is something like Pyle-Pro PQA3100.

also 12" vs 15" drivers and decent wattage to push them.

thank you for your responses. I appreciate the help. A little history I was speaking to my dad about thinking it would be neat to build a speaker. he relied I always have thought that would be fun i have even bought a book on it. so I was thinking a sub would be the best start. my dad has a wood working shop and he is building kitchen cabinets so the wood working should not be a issue. Currently I am trying to figure out what my dad wants. any direction would be nice the room is approximately 15/24 ft with 8 ft ceilings. my first impulse to build would be a THT with a 15" but im not sure my dad is wanting that kind of budget. but would a 250-300w plate amp be enough to power that? if so we might try that I like the pro amp and will go that direction when I build my own stuff but I dont know that my father will justify the extra 100$.

If we do a plate amp i have looked at theDayton Audio SPA250 250W http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-803 or the Yung SD300-6 300W http://www.parts-express.com/pe/showdetl.cfm?partnumber=301-510. I have been reading and its mixed on both people like the yung but it has a filter on it that people say is bad for subs i am a bit confused by this. which amp would work better. if it were just me and i had the bugget I would get a EP-4000 but thats another story.

if you have any advice on anything here any help would be gerat.