"Just a good classic rock sound"? Any good classic rock sound will do, then? Because the classic Sunn sound is rather different from the Ampeg sound, and both are different from, say, the Acoustic sound.
The Beta 15A has a higher Q than the old JBL drivers, so they'll sound rather different on the low end. Additionally, the Beta 15A also has a big peak in the breakup region, so it will sound brighter, less flat than the old JBL's. Both will sound fine, if you like the way they sound, but they will not sound the same. Which one would suit your sound is something only you can say. For me, the JBL's would suit, but the Beta 15A's would not. YMMV.