Why only Sennheiser? For $40 or less there are many other better choices. For example,
the Sony MDR-EX210B for around $19 has 13.5 mm drivers and deep bass, however the bass sounds natural and not exaggerated. The Panasonic RPHJE450 for around $23 has 12.5mm drivers and prouces deep bass that is not exaggerated, as well as decent detail. It is very ergonomically shaped and imo fits great and is very comfortable. It is the IEM I use the most due to ut nice sound and great fit. It is ported though, so isolation isn't great. The Panasonic RPHJE355 for around $18 has more detail than the HJE450, but not as much bass impact since it has a smaller driver. The JVC HAFX40 for around $18 has great detail and good bass, however it is very bright, and some say it has too much treble.
I have the Sennheiser CX300, which is very much like the CX300II. The CX300 is okay, however imo the other earphones I mentioned above sound better than the CX300 and also cost less. The earphones I mentioned above are all more efficient than the CX300II. The CX300II is only around 95 db/mw(113 db/V 16 ohms) vs at least 102 db/mw for the other models I mentioned.