The UE 7 has 3 drivers and a 2-way crossover...the UE 18+ has 6 drivers and a 4-way crossover. The UE 7 has more perceived mid-range, less perceived low mid-range and less high frequency extension than the UE 18+.
While the UE 7 is capable of a louder volume output, the primary difference is the frequency response and the mid-range response. The UE 7 has more prominent mid-range making it very suitable for guitarists, vocalists. The UE 18+ is a shade of color between the UE 7 and UE 11….it does not emphasize the mids like the UE 7 or the lows/highs like the UE 11. The UE 18+ has smooth, clear, and even response with a bit of extra emphasis on the low frequency range.