Reb Yitayningwut- The above mentioned acharonim simply state that there is an additional issur for a kli neginah included in the aveilus of sefirah.
yes it is indeed assur all year round, but on sefira there is an additional stringency.
Why would they say something that something assur is assur. I guess it is like the poskim who assur smoking all yeaR and also discuss whether it is muttar on yom tov. Different Dinim.
Besides the Magen avraham is actually talking about by a seudas Eirusin which during the year there is no issur.
And kemiduma li we already discussed this once http://www.theyeshivaworld.com/coffeeroom/topic/music-in-the-9-days