geez, you really do need help!
im a junior and ive been playing the alto since the 6 grade.
first off, the letter refers to the inner chamber size of the mouthpiece with A being the largest. the larger the chamber size, the darker the sound and better for concert music. the smaller the chamber, the brighter the sound. typically better for marching and jazz music. the number refers to the facing, or distance the reed is from the mouthpiece. 3 is small, 5 is about average, and 7 is large. the larger the facing, the more room the reed has to vibrate, the louder the sound.
when you say you're getting worse, do you mean you're having trouble reading the music?
lower pitches require longer wavelengths. you could be putting too much pressure in the embouchre. try slackening the jaw and don't be afraid to put air throught the horn and belt out a loud low note!
as for the crackly/hissy/spitty sound, try sucking on the mouthpiece to remove the excess moisture. i know it sounds gross, but it works wonders!
i use a B7 mouthpiece with a small chamber during marching season. it has a large facing that gives me volume to be heard across the stadium. i use an A3 during concert season so i don't drown out the rest of the band (our band has less than 20 people, half of which could care less about playing their instrument!)
i commonly use a 2&1/2-3 reed. try sticking with a 2&1/2 for a while and see if anyhting happens.
as for the brand, it doesn't matter because the reeds are made of the same cane poles harvested out of the same field and are cut on the same machines. you're just paying for a brand name! but it's a smart idea to use the ones your director provides b/c i don't enjoy paying for reeds either.
i play a Wexler, but before then i played the nastiest, grodiest school instruments we had. there was seemingly no difference, it just depends on your ability.
as for feeling like you have to use a lot of air,try some breathing execises like holding out a note for as long as you can.
i've never played in jazz, but i've played in concert and marching with marching being my favorite.
hope this helps! peace out! altos 4-eva! ;)